SQL Commands:
SQL
commands are instructions used to communicate with the database to perform
specific task that work with data. SQL commands can be used not only for
searching the database but also to perform various other functions like, for
example, you can create tables, add data to tables, or modify data, drop the
table, set permissions for users. SQL commands are grouped into four major
categories depending on their functionality:
·
Data Definition
Language (DDL) - These SQL commands are used for creating, modifying, and
dropping the structure of database objects. The commands are CREATE, ALTER,
DROP, RENAME, and TRUNCATE.
·
Data Manipulation
Language (DML) - These SQL commands are used for storing, retrieving,
modifying, and deleting data. These commands are SELECT, INSERT, UPDATE, and
DELETE.
·
Transaction Control
Language (TCL) - These SQL commands are used for managing changes affecting
the data. These commands are COMMIT, ROLLBACK, and SAVEPOINT.
·
Data Control
Language (DCL) - These SQL commands are used for providing security to
database objects. These commands are GRANT and REVOKE.
SQL SELECT Statement
The most commonly used SQL
command is SELECT statement. The SQL SELECT statement is used to query or
retrieve data from a table in the database. A query may retrieve information
from specified columns or from all of the columns in the table. To create a
simple SQL SELECT Statement, you must specify the column(s) name and the table
name. The whole query is called SQL SELECT Statement.
Syntax of SQL SELECT
Statement:
SELECT column_list FROM
table-name
[WHERE Clause]
[GROUP BY clause]
[HAVING clause]
[ORDER BY clause];
•table-name is the name of
the table from which the information is retrieved.
•column_list includes one or
more columns from which data is retrieved.
•The code within the
brackets is optional.
database table
student_details;
id first_name last_name age
subject games
100 Rahul Sharma 10 Science
Cricket
101 Anjali Bhagwat 12 Maths
Football
102 Stephen Fleming 09
Science Cricket
103 Shekar Gowda 18 Maths
Badminton
104 Priya Chandra 15
Economics Chess
NOTE: These database tables
are used here for better explanation of SQL commands. In reality, the tables
can have different columns and different data.
For example, consider the
table student_details. To select the first name of all the students the query
would be like:
SELECT first_name FROM
student_details;
NOTE: The commands are not
case sensitive. The above SELECT statement can also be written as "select
first_name from students_details;"
You can also retrieve data
from more than one column. For example, to select first name and last name of
all the students.
SELECT first_name, last_name
FROM student_details;
You can also use clauses
like WHERE, GROUP BY, HAVING, ORDER BY with SELECT statement. We will discuss
these commands in coming chapters.
NOTE: In a SQL SELECT
statement only SELECT and FROM statements are mandatory. Other clauses like
WHERE, ORDER BY, GROUP BY, HAVING are optional.
How to use expressions in
SQL SELECT Statement?
Expressions combine many
arithmetic operators, they can be used in SELECT, WHERE and ORDER BY Clauses of
the SQL SELECT Statement.
Here we will explain how to
use expressions in the SQL SELECT Statement. About using expressions in WHERE
and ORDER BY clause, they will be explained in their respective sections.
The operators are evaluated
in a specific order of precedence, when more than one arithmetic operator is
used in an expression. The order of evaluation is: parentheses, division,
multiplication, addition, and subtraction. The evaluation is performed from the
left to the right of the expression.
For example: If we want to
display the first and last name of an employee combined together, the SQL
Select Statement would be like
SELECT first_name || ' ' || last_name
FROM employee;
Output:
first_name || ' ' ||
last_name
---------------------------------
Rahul Sharma
Anjali Bhagwat
Stephen Fleming
Shekar Gowda
Priya Chandra
You can also provide aliases
as below.
SELECT first_name || ' ' ||
last_name AS emp_name FROM employee;
Output:
emp_name
-------------
Rahul Sharma
Anjali Bhagwat
Stephen Fleming
Shekar Gowda
Priya Chandra
SQL Alias
SQL Aliases are defined for
columns and tables. Basically aliases is created to make the column selected
more readable.
For Example: To select the
first name of all the students, the query would be like:
Aliases for columns:
SELECT first_name AS Name
FROM student_details;
or
SELECT first_name Name FROM
student_details;
In the above query, the
column first_name is given a alias as 'name'. So when the result is displayed
the column name appears as 'Name' instead of 'first_name'.
Output:
Name
-------------
Rahul Sharma
Anjali Bhagwat
Stephen Fleming
Shekar Gowda
Priya Chandra
Aliases for tables:
SELECT s.first_name FROM
student_details s;
In the above query, alias
's' is defined for the table student_details and the column first_name is
selected from the table.
Aliases is more useful when
•There are more than one
tables involved in a query,
•Functions are used in the
query,
•The column names are big or
not readable,
•More than one columns are
combined together
SQL WHERE Clause
The WHERE Clause is used
when you want to retrieve specific information from a table excluding other
irrelevant data. For example, when you want to see the information about
students in class 10th only then you do need the information about the students
in other class. Retrieving information about all the students would increase
the processing time for the query.
So SQL offers a feature
called WHERE clause, which we can use to restrict the data that is retrieved.
The condition you provide in the WHERE clause filters the rows retrieved from
the table and gives you only those rows which you expected to see. WHERE clause
can be used along with SELECT, DELETE, UPDATE statements.
Syntax of SQL WHERE Clause:
WHERE {column or expression}
comparison-operator value
Syntax for a WHERE clause
with Select statement is:
SELECT column_list FROM
table-name
WHERE condition;
•column or expression - Is
the column of a table or a expression
•comparison-operator -
operators like = < > etc.
•value - Any user value or a
column name for comparison
For Example: To find the
name of a student with id 100, the query would be like:
SELECT first_name, last_name
FROM student_details
WHERE id = 100;
Comparison Operators and
Logical Operators are used in WHERE Clause. These operators are discussed in
the next chapter.
NOTE: Aliases defined for the columns in the SELECT
statement cannot be used in the WHERE clause to set conditions. Only aliases
created for tables can be used to reference the columns in the table.
How to use expressions in
the WHERE Clause?
Expressions can also be used
in the WHERE clause of the SELECT statement.
For example: Lets consider
the employee table. If you want to display employee name, current salary, and a
20% increase in the salary for only those products where the percentage increase
in salary is greater than 30000, the SELECT statement can be written as shown
below
SELECT name, salary,
salary*1.2 AS new_salary FROM employee
WHERE salary*1.2 > 30000;
Output:
name salary new_salary
----------- ----------
----------------
Hrithik 35000 37000
Harsha 35000 37000
Priya 30000 360000
NOTE: Aliases defined in the SELECT Statement can’t be used in WHERE Clause.
SQL Operators
There are two type of
Operators, namely Comparison Operators and Logical Operators. These operators are
used mainly in the WHERE clause, HAVING clause to filter the data to be
selected.
Comparison Operators:
Comparison operators are
used to compare the column data with specific values in a condition.
Comparison Operators are
also used along with the SELECT statement to filter data based on specific
conditions.
The below table describes
each comparison operator.
Comparison Operators
Description
= equal to
<>, != is not equal to
< less than
> greater than
>= greater than or equal to
<= less than or equal to
Logical Operators:
There are three Logical
Operators namely AND, OR and NOT.
Logical operators are
discussed in detail in the next section
SQL Logical Operators
There are three Logical Operators
namely, AND, OR, and NOT. These operators compare two conditions at a time to
determine whether a row can be selected for the output. When retrieving data
using a SELECT statement, you can use logical operators in the WHERE clause,
which allows you to combine more than one condition.
SQL Comparison Keywords
There are other comparison
keywords available in sql which are used to enhance the search capabilities of
a sql query. They are "IN", "BETWEEN...AND", "IS
NULL", "LIKE".
Comparision Operators
Description
LIKE column value is similar
to specified character(s).
IN column value is equal to
any one of a specified set of values.
BETWEEN...AND column value
is between two values, including the end values specified in the range.
IS NULL column value does
not exist.
SQL LIKE Operator
The LIKE operator is used to
list all rows in a table whose column values match a specified pattern. It is
useful when you want to search rows to match a specific pattern, or when you do
not know the entire value. For this purpose we use a wildcard character '%'.
For example: To select all
the students whose name begins with 'S'
SELECT first_name, last_name
FROM student_details
WHERE first_name LIKE 'S%';
The output would be similar
to:
first_name last_name
------------- -------------
Stephen Fleming
Shekar Gowda
The above select statement
searches for all the rows where the first letter of the column first_name is
'S' and rest of the letters in the name can be any character.
There is another wildcard
character you can use with LIKE operator. It is the underscore character, ' _ '
. In a search string, the underscore signifies a single character.
For example: to display all
the names with 'a' second character,
SELECT first_name, last_name
FROM student_details
WHERE first_name LIKE '_a%';
The output would be similar
to:
first_name last_name
------------- -------------
Rahul Sharma
NOTE:Each underscore act
as a placeholder for only one character. So you can use more than one
underscore. Eg: ' __i% '-this has two underscores towards the left, 'S__j%' -
this has two underscores between character 'S' and 'i'.
SQL BETWEEN ... AND Operator
The operator BETWEEN and
AND, are used to compare data for a range of values.
For Example: to find the
names of the students between age 10 to 15 years, the query would be like,
SELECT first_name,
last_name, age
FROM student_details
WHERE age BETWEEN 10 AND 15;
The output would be similar
to:
first_name last_name age
------------- -------------
------
Rahul Sharma 10
Anajali Bhagwat 12
Shekar Gowda 15
SQL IN Operator:
The IN operator is used when
you want to compare a column with more than one value. It is similar to an OR
condition.
For example: If you want to
find the names of students who are studying either Maths or Science, the query
would be like,
SELECT first_name,
last_name, subject
FROM student_details
WHERE subject IN ('Maths',
'Science');
The output would be similar
to:
first_name last_name subject
------------- -------------
----------
Anajali Bhagwat Maths
Shekar Gowda Maths
Rahul Sharma Science
Stephen Fleming Science
You can include more
subjects in the list like ('maths','science','history')
NOTE:The data used to compare is case sensitive.
SQL IS NULL Operator
A column value is NULL if it
does not exist. The IS NULL operator is used to display all the rows for
columns that do not have a value.
For Example: If you want to
find the names of students who do not participate in any games, the query would
be as given below
SELECT first_name, last_name
FROM student_details
WHERE games IS NULL
There would be no output as
we have every student participate in a game in the table student_details, else
the names of the students who do not participate in any games would be
displayed.
SQL ORDER BY
The ORDER BY clause is used
in a SELECT statement to sort results either in ascending or descending order.
Oracle sorts query results in ascending order by default.
Syntax for using SQL ORDER
BY clause to sort data is:
SELECT column-list
FROM table_name [WHERE condition]
[ORDER BY column1 [,
column2, .. columnN] [DESC]];
database table
"employee";
id name
dept age salary
location
100 Ramesh Electrical 24
25000 Bangalore
101 Hrithik Electronics 28 35000 Bangalore
102 Harsha Aeronautics 28
35000 Mysore
103 Soumya Electronics 22
20000 Bangalore
104 Priya InfoTech 25 30000
Mangalore
For Example: If you want to
sort the employee table by salary of the employee, the sql query would be.
SELECT name, salary FROM
employee ORDER BY salary;
The output would be like
name salary
---------- ----------
Soumya 20000
Ramesh 25000
Priya 30000
Hrithik 35000
Harsha 35000
The query first sorts the
result according to name and then displays it.
You can also use more than
one column in the ORDER BY clause.
If you want to sort the
employee table by the name and salary, the query would be like,
SELECT name, salary FROM
employee ORDER BY name, salary;
The output would be like:
name salary
------------- -------------
Soumya 20000
Ramesh 25000
Priya 30000
Harsha 35000
Hrithik 35000
NOTE:The columns specified in ORDER BY clause should
be one of the columns selected in the SELECT column list.
You can represent the
columns in the ORDER BY clause by specifying the position of a column in the
SELECT list, instead of writing the column name.
The above query can also be
written as given below,
SELECT name, salary FROM
employee ORDER BY 1, 2;
By default, the ORDER BY Clause sorts data in
ascending order. If you want to sort the data in descending order, you must
explicitly specify it as shown below.
SELECT name, salary
FROM employee
ORDER BY name, salary DESC;
The above query sorts only
the column 'salary' in descending order and the column 'name' by ascending
order.
If you want to select both
name and salary in descending order, the query would be as given below.
SELECT name, salary
FROM employee
ORDER BY name DESC, salary
DESC;
How to use expressions in
the ORDER BY Clause?
Expressions in the ORDER BY
clause of a SELECT statement.
For example: If you want to
display employee name, current salary, and a 20% increase in the salary for
only those employees for whom the percentage increase in salary is greater than
30000 and in descending order of the increased price, the SELECT statement can
be written as shown below
SELECT name, salary,
salary*1.2 AS new_salary
FROM employee
WHERE salary*1.2 > 30000
ORDER BY new_salary DESC;
The output for the above
query is as follows.
name salary new_salary
---------- ----------
-------------
Hrithik 35000 37000
Harsha 35000 37000
Priya 30000 36000
NOTE:Aliases defined in the SELECT Statement can be
used in ORDER BY Clause.
SQL GROUP Functions
Group functions are built-in
SQL functions that operate on groups of rows and return one value for the
entire group. These functions are: COUNT, MAX, MIN, AVG, SUM, DISTINCT
SQL COUNT (): This function
returns the number of rows in the table that satisfies the condition specified
in the WHERE condition. If the WHERE condition is not specified, then the query
returns the total number of rows in the table.
For Example: If you want the
number of employees in a particular department, the query would be:
SELECT COUNT (*) FROM
employee
WHERE dept = 'Electronics';
The output would be '2'
rows.
If you want the total number
of employees in all the department, the query would take the form:
SELECT COUNT (*) FROM
employee;
The output would be '5' rows.
SQL DISTINCT(): This
function is used to select the distinct rows.
For Example: If you want to
select all distinct department names from employee table, the query would be:
SELECT DISTINCT dept FROM
employee;
To get the count of
employees with unique name, the query would be:
SELECT COUNT (DISTINCT name)
FROM employee;
SQL MAX(): This function is
used to get the maximum value from a column.
To get the maximum salary
drawn by an employee, the query would be:
SELECT MAX (salary) FROM
employee;
SQL MIN(): This function is
used to get the minimum value from a column.
To get the minimum salary
drawn by an employee, he query would be:
SELECT MIN (salary) FROM
employee;
SQL AVG(): This function is
used to get the average value of a numeric column.
To get the average salary,
the query would be
SELECT AVG (salary) FROM
employee;
SQL SUM(): This function is
used to get the sum of a numeric column
To get the total salary
given out to the employees,
SELECT SUM (salary) FROM
employee;
SQL GROUP BY Clause
The SQL GROUP BY Clause is
used along with the group functions to retrieve data grouped according to one
or more columns.
For Example: If you want to
know the total amount of salary spent on each department, the query would be:
SELECT dept, SUM (salary)
FROM employee
GROUP BY dept;
The output would be like:
dept salary
----------------
--------------
Electrical 25000
Electronics 55000
Aeronautics 35000
InfoTech 30000
NOTE: The group by clause should contain all the
columns in the select list expect those used along with the group functions.
SELECT location, dept, SUM
(salary)
FROM employee
GROUP BY location, dept;
The output would be like:
location dept
salary
-------------
--------------- -----------
Bangalore Electrical 25000
Bangalore Electronics 55000
Mysore Aeronautics 35000
Mangalore InfoTech 30000
SQL HAVING Clause
Having clause is used to
filter data based on the group functions. This is similar to WHERE condition
but is used with group functions. Group functions cannot be used in WHERE
Clause but can be used in HAVING clause.
For Example: If you want to
select the department that has total salary paid for its employees more than
25000, the sql query would be like;
SELECT dept, SUM (salary)
FROM employee
GROUP BY dept
HAVING SUM (salary) >
25000
The output would be like:
dept salary
------------- -------------
Electronics 55000
Aeronautics 35000
InfoTech 30000
When WHERE, GROUP BY and HAVING
clauses are used together in a SELECT statement, the WHERE clause is processed
first, then the rows that are returned after the WHERE clause is executed are
grouped based on the GROUP BY clause. Finally, any conditions on the group
functions in the HAVING clause are applied to the grouped rows before the final
output is displayed.
SQL INSERT Statement
The INSERT Statement is used
to add new rows of data to a table.
We can insert data to a
table in two ways,
1) Inserting the data
directly to a table.
Syntax for SQL INSERT is:
INSERT INTO TABLE_NAME
[ (col1, col2,
col3,...colN)]
VALUES (value1, value2,
value3,...valueN);
•col1, col2,...colN -- the
names of the columns in the table into which you want to insert data.
While inserting a row, if
you are adding value for all the columns of the table you need not specify the
column(s) name in the sql query. But you need to make sure the order of the
values is in the same order as the columns in the table. The sql insert query
will be as follows
INSERT INTO TABLE_NAME
VALUES (value1, value2,
value3,...valueN);
For Example: If you want to
insert a row to the employee table, the query would be like,
INSERT INTO employee (id,
name, dept, age, salary location) VALUES (105, 'Srinath', 'Aeronautics', 27,
33000);
NOTE:When adding a row, only
the characters or date values should be enclosed with single quotes.
If you are inserting data to
all the columns, the column names can be omitted. The above insert statement
can also be written as,
INSERT INTO employee
VALUES (105, 'Srinath',
'Aeronautics', 27, 33000);
Inserting data to a table
through a select statement.
Syntax for SQL INSERT is:
INSERT INTO table_name
[(column1, column2, ...
columnN)]
SELECT column1, column2,
...columnN
FROM table_name [WHERE
condition];
For Example: To insert a row
into the employee table from a temporary table, the sql insert query would be
like,
INSERT INTO employee (id,
name, dept, age, salary location) SELECT emp_id, emp_name, dept, age, salary,
location
FROM temp_employee;
If you are inserting data to
all the columns, the above insert statement can also be written as,
INSERT INTO employee
SELECT * FROM temp_employee;
NOTE:We have assumed the
temp_employee table has columns emp_id, emp_name, dept, age, salary, location
in the above given order and the same datatype.
IMPORTANT NOTE:
1) When adding a new row,
you should ensure the datatype of the value and the column matches
2) You follow the integrity
constraints, if any, defined for the table.
SQL
UPDATE Statement
The UPDATE Statement is used
to modify the existing rows in a table.
The Syntax for SQL UPDATE
Command is:
UPDATE table_name
SET column_name1 = value1,
column_name2 = value2, ...
[WHERE condition]
•table_name - the table name
which has to be updated.
•column_name1,
column_name2.. - the columns that gets changed.
•value1, value2... - are the
new values.
NOTE:In the Update
statement, WHERE clause identifies the rows that get affected. If you do not
include the WHERE clause, column values for all the rows get affected.
For Example: To update the
location of an employee, the sql update query would be like,
UPDATE employee
SET location ='Mysore'
WHERE id = 101;
To change the salaries of
all the employees, the query would be,
UPDATE employee
SET salary = salary +
(salary * 0.2);
SQL Delete Statement
The DELETE Statement is used
to delete rows from a table.
The Syntax of a SQL DELETE
statement is:
DELETE FROM table_name
[WHERE condition];
•table_name -- the table
name which has to be updated.
NOTE:The WHERE clause in the
sql delete command is optional and it identifies the rows in the column that
gets deleted. If you do not include the WHERE clause all the rows in the table
is deleted, so be careful while writing a DELETE query without WHERE clause.
For Example: To delete an
employee with id 100 from the employee table, the sql delete query would be
like,
DELETE FROM employee WHERE
id = 100;
To delete all the rows from
the employee table, the query would be like,
DELETE FROM employee;
SQL TRUNCATE Statement
The SQL TRUNCATE command is
used to delete all the rows from the table and free the space containing the
table.
Syntax to TRUNCATE a table:
TRUNCATE TABLE table_name;
For Example: To delete all
the rows from employee table, the query would be like,
TRUNCATE TABLE employee;
Difference between DELETE and TRUNCATE Statements:
DELETE Statement: This
command deletes only the rows from the table based on the condition given in
the where clause or deletes all the rows from the table if no condition is
specified. But it does not free the space containing the table.
TRUNCATE statement: This
command is used to delete all the rows from the table and free the space
containing the table.
SQL DROP Statement:
The SQL DROP command is used
to remove an object from the database. If you drop a table, all the rows in the
table is deleted and the table structure is removed from the database. Once a
table is dropped we cannot get it back, so be careful while using RENAME
command. When a table is dropped all the references to the table will not be
valid.
Syntax to drop a sql table
structure:
DROP TABLE table_name;
For Example: To drop the
table employee, the query would be like
DROP TABLE employee;
Difference between DROP and TRUNCATE Statement:
If a table is dropped, all
the relationships with other tables will no longer be valid, the integrity
constraints will be dropped, grant or access privileges on the table will also
be dropped, if want use the table again it has to be recreated with the integrity
constraints, access privileges and the relationships with other tables should
be established again. But, if a table is truncated, the table structure remains
the same, therefore any of the above problems will not exist.
SQL CREATE TABLE Statement
The CREATE TABLE Statement
is used to create tables to store data. Integrity Constraints like primary key,
unique key, foreign key can be defined for the columns while creating the
table. The integrity constraints can be defined at column level or table level.
The implementation and the syntax of the CREATE Statements differs for
different RDBMS.
The Syntax for the CREATE
TABLE Statement is:
CREATE TABLE table_name
(column_name1 datatype,
column_name2 datatype,
... column_nameN datatype
);
•table_name - is the name of
the table.
•column_name1,
column_name2.... - is the name of the columns
•datatype - is the datatype
for the column like char, date, number etc.
For Example: If you want to
create the employee table, the statement would be like,
CREATE TABLE employee
( id number(5),
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10)
);
In Oracle database, the
datatype for an integer column is represented as "number". In Sybase
it is represented as "int".
Oracle provides another way
of creating a table.
CREATE TABLE temp_employee
SELECT * FROM employee
In the above statement,
temp_employee table is created with the same number of columns and datatype as
employee table.
SQL ALTER TABLE Statement
The SQL ALTER TABLE command
is used to modify the definition (structure) of a table by modifying the
definition of its columns. The ALTER command is used to perform the following
functions.
1) Add, drop, modify table
columns
2) Add and drop constraints
3) Enable and Disable
constraints
e.g
ALTER TABLE t2 ADD id number(10) PRIMARY KEY;
ALTER TABLE t2 DROP COLUMN c;
To rename the table from t1 to t2:
ALTER TABLE t1 RENAME t2;
To change column columnName from INTEGER to String(10) (leaving the
name the same), and to change column column2 from CHAR(10) to CHAR(20) as well as renaming it from column2 to newColumn2:
ALTER TABLE t2 MODIFY ColName String(10),
CHANGE column2 newColumn2 CHAR(20);
ALTER TABLE t2 ADD d TIMESTAMP;
SQL Integrity Constraints
Integrity Constraints are
used to apply business rules for the database tables.
The constraints available in
SQL are Foreign Key, Not Null, Unique, Check.
Constraints can be defined
in two ways
1) The constraints can be
specified immediately after the column definition. This is called column-level
definition.
2) The constraints can be
specified after all the columns are defined. This is called table-level
definition.
1) SQL Primary key:
This constraint defines a
column or combination of columns which uniquely identifies each row in the
table.
Syntax to define a Primary
key at column level:
column name datatype
[CONSTRAINT constraint_name] PRIMARY KEY
Syntax to define a Primary
key at table level:
[CONSTRAINT constraint_name]
PRIMARY KEY (column_name1,column_name2,..)
•column_name1, column_name2
are the names of the columns which define the primary Key.
•The syntax within the
bracket i.e. [CONSTRAINT constraint_name] is optional.
For Example: To create an
employee table with Primary Key constraint, the query would be like.
Primary Key at table level:
CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10)
);
or
CREATE TABLE employee
( id number(5) CONSTRAINT
emp_id_pk PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10)
);
Primary Key at table level:
CREATE TABLE employee
( id number(5),
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10),
CONSTRAINT emp_id_pk PRIMARY
KEY (id)
);
2) SQL Foreign key or
Referential Integrity:
This constraint identifies
any column referencing the PRIMARY KEY in another table. It establishes a
relationship between two columns in the same table or between different tables.
For a column to be defined as a Foreign Key, it should be a defined as a Primary
Key in the table which it is referring. One or more columns can be defined as
Foreign key.
Syntax to define a Foreign
key at column level:
[CONSTRAINT constraint_name]
REFERENCES Referenced_Table_name(column_name)
Syntax to define a Foreign
key at table level:
[CONSTRAINT constraint_name]
FOREIGN KEY(column_name) REFERENCES referenced_table_name(column_name);
For Example:
1) Lets use the
"product" table and "order_items".
Foreign Key at column level:
CREATE TABLE product
( product_id number(5)
CONSTRAINT pd_id_pk PRIMARY KEY,
product_name char(20),
supplier_name char(20),
unit_price number(10)
);
CREATE TABLE order_items
( order_id number(5)
CONSTRAINT od_id_pk PRIMARY KEY,
product_id number(5)
CONSTRAINT pd_id_fk REFERENCES, product(product_id),
product_name char(20),
supplier_name char(20),
unit_price number(10)
);
Foreign Key at table level:
CREATE TABLE order_items
( order_id number(5) ,
product_id number(5),
product_name char(20),
supplier_name char(20),
unit_price number(10)
CONSTRAINT od_id_pk PRIMARY
KEY(order_id),
CONSTRAINT pd_id_fk FOREIGN
KEY(product_id) REFERENCES product(product_id)
);
2) If the employee table has
a 'mgr_id' i.e, manager id as a foreign key which references primary key 'id'
within the same table, the query would be like,
CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
mgr_id number(5) REFERENCES
employee(id),
salary number(10),
location char(10)
);
3) SQL Not Null
Constraint :
This constraint ensures all
rows in the table contain a definite value for the column which is specified as
not null. Which means a null value is not allowed.
Syntax to define a Not Null
constraint:
[CONSTRAINT constraint name]
NOT NULL
For Example: To create a
employee table with Null value, the query would be like
CREATE TABLE employee
( id number(5),
name char(20) CONSTRAINT
nm_nn NOT NULL,
dept char(10),
age number(2),
salary number(10),
location char(10)
);
Note :
Not Null can’t be defined at
table level
4) SQL Unique Key:
This constraint ensures that
a column or a group of columns in each row have a distinct value. A column(s)
can have a null value but the values cannot be duplicated.
Syntax to define a Unique key
at column level:
[CONSTRAINT constraint_name]
UNIQUE
Syntax to define a Unique
key at table level:
[CONSTRAINT constraint_name]
UNIQUE(column_name)
For Example: To create an
employee table with Unique key, the query would be like,
Unique Key at column level:
CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10) UNIQUE
);
or
CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10) CONSTRAINT
loc_un UNIQUE
);
Unique Key at table level:
CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10),
CONSTRAINT loc_un UNIQUE(location)
);
5) SQL Check Constraint :
This constraint defines a
business rule on a column. All the rows must satisfy this rule. The constraint
can be applied for a single column or a group of columns.
Syntax to define a Check
constraint:
[CONSTRAINT constraint_name]
CHECK (condition)
For Example: In the employee
table to select the gender of a person, the query would be like
Check Constraint at column
level:
CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
gender char(1) CHECK (gender
in ('M','F')),
salary number(10),
location char(10)
);
Check Constraint at table
level:
CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
gender char(1),
salary number(10),
location char(10),
CONSTRAINT gender_ck CHECK
(gender in ('M','F'))
);
What is the difference
between column level constraints and
table level constraints?
· Column level constraints are
applicable to that column
only, whereas table level conatraints are used to define
composite keys like primary key for the combination of two
or more columns in a table
only, whereas table level conatraints are used to define
composite keys like primary key for the combination of two
or more columns in a table
· column level
constraints contain all types of constraints
(like, not null,primary key,foregin key,unique).but table
level except not null constraint its supports all
constraints.
(like, not null,primary key,foregin key,unique).but table
level except not null constraint its supports all
constraints.
SQL Joins
SQL Joins are used to relate
information in different tables. A Join condition is a part of the sql query
that retrieves rows from two or more tables. A SQL Join condition is used in
the SQL WHERE Clause of select, update, delete statements.
The Syntax for joining two
tables is:
SELECT col1, col2, col3...
FROM table_name1,
table_name2
WHERE table_name1.col2 =
table_name2.col1;
If a sql join condition is
omitted or if it is invalid the join operation will result in a Cartesian
product. The Cartesian product returns a number of rows equal to the product of
all rows in all the tables being joined. For example, if the first table has 20
rows and the second table has 10 rows, the result will be 20 * 10, or 200 rows.
This query takes a long time to execute.
|
Employee table
|
|
|
LastName
|
DepartmentID
|
|
Rafferty
|
31
|
|
Jones
|
33
|
|
Heisenberg
|
33
|
|
Robinson
|
34
|
|
Smith
|
34
|
|
John
|
NULL
|
|
Department table
|
|
|
DepartmentID
|
DepartmentName
|
|
31
|
Sales
|
|
33
|
Engineering
|
|
34
|
Clerical
|
|
35
|
Marketing
|
Cross
join
CROSS JOIN returns the Cartesian
product of rows from tables in the join. In other words, it will produce
rows which combine each row from the first table with each row from the second
table
Example
of an explicit cross join:
SELECT *
FROM employee CROSS JOIN
department;
Example
of an implicit cross join:
SELECT *
FROM employee,
department;
Inner
join
This is the most used join in the SQL. this join returns only
those records/rows that match/exists in both the database tables.
SQL specifies two different syntactical ways to express joins:
"explicit join notation" and "implicit join notation".
The
"explicit join notation" uses the JOIN keyword, optionally
preceded by the INNER keyword, to specify
the table to join, and the ON keyword to specify
the condition for the join, as in the following example:
SELECT *
FROM employee INNER JOIN
department
ON employee.DepartmentID =
department.DepartmentID;
Using implicit join notation:
SELECT *
FROM employee, department
WHERE employee.DepartmentID = department.DepartmentID;
Inner Join type
1. Equi join
2. Non equi join
3. Natural join
Equi-join
An equi-join is a specific type of comparator-based join,
that uses only equality comparisons in the condition
e.g =
Non Equi-join
An equi-join is a specific type of comparator-based join,
that uses other than equality comparisons in the condition
e.g > , < , >= ,<= <>
Natural Join
A natural join is a type of equi-join where the join
condition arises implicitly by comparing
all columns in both tables that have the same column-names in the joined
tables. The resulting joined table contains only one column for each pair of
equally named columns.
SELECT *
FROM employee NATURAL
JOIN department;
As
with the explicit USING clause, only one
DepartmentID column occurs in the joined table, with no qualifier:
|
DepartmentID
|
Employee.LastName
|
Department.DepartmentName
|
|
34
|
Smith
|
Clerical
|
|
33
|
Jones
|
Engineering
|
|
34
|
Robinson
|
Clerical
|
|
33
|
Heisenberg
|
Engineering
|
|
31
|
Rafferty
|
Sales
|
Outer
join
An outer join does not require each record in the two joined
tables to have a matching record. The joined table retains each record—even if
no other matching record exists.
Types of Outer Join
1. Left Outer Join (All rows from left table)
2. Right outer join (All rows from right table)
3. Full outer join (All rows from both table)
Left
outer join
The result of a left outer join (or simply left join)
for tables A and B always contains all records of the "left" table
(A), even if the join-condition does not find any matching record in the
"right" table (B). This means that if the
ON clause matches 0 (zero) records
in B (for a given record in A), the join will still return a row in the result
(for that record)—but with NULL in each column from B. A left outer join
returns all the values from an inner join plus all values in the left table
that do not match to the right table.SELECT *
FROM employee LEFT OUTER JOIN department
ON employee.DepartmentID = department.DepartmentID;
SELECT *
FROM employee, department
WHERE employee.DepartmentID = department.DepartmentID(+)
|
Employee.LastName
|
Employee.DepartmentID
|
Department.DepartmentName
|
Department.DepartmentID
|
|
Jones
|
33
|
Engineering
|
33
|
|
Rafferty
|
31
|
Sales
|
31
|
|
Robinson
|
34
|
Clerical
|
34
|
|
Smith
|
34
|
Clerical
|
34
|
|
John
|
NULL
|
NULL
|
NULL
|
|
Heisenberg
|
33
|
Engineering
|
33
|
Right
outer join
A right outer join (or right join) closely resembles a
left outer join, except with the treatment of the tables reversed. Every row
from the "right" table (B) will appear in the joined table at least
once. If no matching row from the "left" table (A) exists, NULL will
appear in columns from A for those records that have no match in B.
SELECT *
FROM employee RIGHT OUTER JOIN department
ON employee.DepartmentID = department.DepartmentID;
SELECT *
FROM employee, department
WHERE employee.DepartmentID(+) = department.DepartmentID
|
Employee.LastName
|
Employee.DepartmentID
|
Department.DepartmentName
|
Department.DepartmentID
|
|
Smith
|
34
|
Clerical
|
34
|
|
Jones
|
33
|
Engineering
|
33
|
|
Robinson
|
34
|
Clerical
|
34
|
|
Heisenberg
|
33
|
Engineering
|
33
|
|
Rafferty
|
31
|
Sales
|
31
|
|
NULL
|
NULL
|
Marketing
|
35
|
Full
outer join
Conceptually, a full outer join combines the effect of
applying both left and right outer joins. Where records in the FULL OUTER
JOINed tables do not match, the result set will have NULL values for every
column of the table that lacks a matching row. For those records that do match,
a single row will be produced in the result set (containing fields populated
from both tables).
SELECT *
FROM employee FULL OUTER JOIN department
ON employee.DepartmentID = department.DepartmentID;
|
Employee.LastName
|
Employee.DepartmentID
|
Department.DepartmentName
|
Department.DepartmentID
|
|
Smith
|
34
|
Clerical
|
34
|
|
Jones
|
33
|
Engineering
|
33
|
|
Robinson
|
34
|
Clerical
|
34
|
|
John
|
NULL
|
NULL
|
NULL
|
|
Heisenberg
|
33
|
Engineering
|
33
|
|
Rafferty
|
31
|
Sales
|
31
|
|
NULL
|
NULL
|
Self-join
A self-join is joining a table to itself.[
e.g
A query to find all pairings of two employees in the same country is
desired. If there were two separate tables for employees and a query which
requested employees in the first table having the same country as employees in
the second table, a normal join operation could be used to find the answer
table. However, all the employee information is contained within a single large
table
|
Employee Table
|
|||
|
EmployeeID
|
LastName
|
Country
|
DepartmentID
|
|
123
|
Rafferty
|
Australia
|
31
|
|
124
|
Jones
|
Australia
|
33
|
|
145
|
Heisenberg
|
Australia
|
33
|
|
201
|
Robinson
|
United States
|
34
|
|
305
|
Smith
|
Germany
|
34
|
|
306
|
John
|
Germany
|
NULL
|
An
example solution query could be as follows:
SELECT F.EmployeeID,
F.LastName, S.EmployeeID, S.LastName, F.Country
FROM Employee F INNER
JOIN Employee S ON F.Country = S.Country
WHERE F.EmployeeID <
S.EmployeeID
ORDER BY F.EmployeeID,
S.EmployeeID;
Which
results in the following table being generated.
|
Employee Table after Self-join by
Country
|
||||
|
EmployeeID
|
LastName
|
EmployeeID
|
LastName
|
Country
|
|
123
|
Rafferty
|
124
|
Jones
|
Australia
|
|
123
|
Rafferty
|
145
|
Heisenberg
|
Australia
|
|
124
|
Jones
|
145
|
Heisenberg
|
Australia
|
|
305
|
Smith
|
306
|
John
|
Germany
|
SQL Views
A VIEW is a virtual table,
through which a selective portion of the data from one or more tables can be
seen. Views do not contain data of their own. They are used to restrict access
to the database or to hide data complexity. A view is stored as a SELECT
statement in the database. DML operations on a view like INSERT, UPDATE, DELETE
affects the data in the original table upon which the view is based.
The Syntax to create a sql
view is
CREATE VIEW view_name
AS
SELECT column_list
FROM table_name [WHERE
condition];
•view_name is the name of
the VIEW.
•The SELECT statement is
used to define the columns and rows that you want to display in the view.
For Example: to create a
view on the product table the sql query would be like
CREATE VIEW view_product
AS
SELECT product_id,
product_name
FROM product;
Consider the CUSTOMERS table
having the following records:
+----+----------+-----+-----------+----------+
|
ID | NAME
| AGE |
ADDRESS | SALARY |
+----+----------+-----+-----------+----------+
| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
| 2 | Khilan | 25 | Delhi | 1500.00 |
| 3 | kaushik | 23 | Kota | 2000.00 |
| 4 | Chaitali | 25 | Mumbai | 6500.00 |
| 5 | Hardik | 27 | Bhopal | 8500.00 |
| 6 | Komal | 22 | MP | 4500.00 |
| 7 | Muffy | 24 | Indore | 10000.00 |
+----+----------+-----+-----------+----------+
Now, following is the example
to create a view from CUSTOMERS table. This view would be used to have customer
name and age from CUSTOMERS table:
SQL > CREATE VIEW CUSTOMERS_VIEW AS
SELECT name, age
FROM CUSTOMERS;
Now, you can query
CUSTOMERS_VIEW in similar way as you query an actual table. Following is the
example:
SQL > SELECT * FROM
CUSTOMERS_VIEW;
This would produce the
following result:
+----------+-----+
|
name |
age |
+----------+-----+
|
Ramesh
|
32 |
|
Khilan
|
25 |
|
kaushik | 23 |
|
Chaitali | 25 |
|
Hardik
|
27 |
|
Komal
|
22 |
|
Muffy
|
24 |
+----------+-----+
Updating a View
Rules for Update View
A view can be updated under
certain conditions:
·
The SELECT clause may not contain the keyword DISTINCT.
·
The SELECT clause may not contain summary functions.
·
The SELECT clause may not contain set functions.
·
The SELECT clause may not contain set operators.
·
The SELECT clause may not contain an ORDER BY clause.
·
The FROM clause may not contain multiple tables.
·
The WHERE clause may not contain subqueries.
·
The query may not contain GROUP BY or HAVING.
·
Calculated columns may not be updated.
·
All NOT NULL columns from the base table must be included in the
view in order for the INSERT query to function.
SQL > UPDATE CUSTOMERS_VIEW
SET AGE = 35
WHERE name='Ramesh';
Deleting Rows into a View:
SQL > DELETE FROM CUSTOMERS_VIEW
WHERE age = 22;
Dropping Views:
DROP VIEW view_name;
SQL Subquery
Subquery or Inner query or
Nested query is a query in a query. A subquery is usually added in the WHERE
Clause of the sql statement. Most of the time, a subquery is used when you know
how to search for a value using a SELECT statement, but do not know the exact value.
Subqueries are an alternate
way of returning data from multiple tables.
Subqueries can be used with
the following sql statements along with the comparision operators like =, <,
>, >=, <= etc.
•SELECT
•INSERT
•UPDATE
•DELETE
For Example:
1) Usually, a subquery
should return only one record, but sometimes it can also return multiple
records when used with operators like IN, NOT IN in the where clause. The query
would be like,
SELECT first_name,
last_name, subject
FROM student_details
WHERE games NOT IN
('Cricket', 'Football');
The output would be similar
to:
first_name last_name subject
------------- -------------
----------
Shekar Gowda Badminton
Priya Chandra Chess
2) Lets consider the
student_details table which we have used earlier. If you know the name of the
students who are studying science subject, you can get their id's by using this
query below,
SELECT id, first_name
FROM student_details
WHERE first_name IN
('Rahul', 'Stephen');
but, if you do not know
their names, then to get their id's you need to write the query in this manner,
SELECT id, first_name
FROM student_details
WHERE first_name IN (SELECT
first_name
FROM student_details
WHERE subject= 'Science');
Output:
id first_name
-------- -------------
100 Rahul
102 Stephen
In the above sql statement,
first the inner query is processed first and then the outer query is processed.
3) Subquery can be used with
INSERT statement to add rows of data from one or more tables to another table.
Lets try to group all the students who study Maths in a table 'maths_group'.
INSERT INTO maths_group(id,
name)
SELECT id, first_name || ' '
|| last_name
FROM student_details WHERE
subject= 'Maths'
4) A subquery can be used in
the SELECT statement as follows. Lets use the product and order_items table
defined in the sql_joins section.
select p.product_name,
p.supplier_name, (select order_id from order_items where product_id = 101) as
order_id from product p where p.product_id = 101
product_name supplier_name
order_id
------------------
------------------ ----------
Television Onida 5103
Correlated Subquery
A query is called correlated
subquery when both the inner query and the outer query are interdependent. For
every row processed by the inner query, the outer query is processed as well.
The inner query depends on the outer query before it can be processed.
SELECT p.product_name FROM
product p
WHERE p.product_id = (SELECT
o.product_id FROM order_items o
WHERE o.product_id =
p.product_id);
NOTE:
1) You can nest as many
queries you want but it is recommended not to nest more than 16 subqueries in
oracle.
2) If a subquery is not
dependent on the outer query it is called a non-correlated subquery.
SQL Index
Index in sql is created on
existing tables to retrieve the rows quickly.
When there are thousands of
records in a table, retrieving information will take a long time. Therefore
indexes are created on columns which are accessed frequently, so that the
information can be retrieved quickly. Indexes can be created on a single column
or a group of columns. When a index is created, it first sorts the data and
then it assigns a ROWID for each row.
Syntax to create Index:
CREATE INDEX index_name
ON table_name
(column_name1,column_name2...);
Syntax to create SQL unique
Index:
ATE UNIQUE INDEX index_name
ON table_name
(column_name1,column_name2...);
•index_name is the name of
the INDEX.
•table_name is the name of
the table to which the indexed column belongs.
•column_name1,
column_name2.. is the list of columns which make up the INDEX.
In Oracle there are two
types of SQL index namely, implicit and explicit.
Implicit Indexes:
They are created when a
column is explicity defined with PRIMARY KEY, UNIQUE KEY Constraint.
Explicit Indexes:
They are created using the
"create index.. " syntax.
NOTE:
1) Even though sql indexes
are created to access the rows in the table quickly, they slow down DML
operations like INSERT, UPDATE, DELETE on the table, because the indexes and
tables both are updated along when a DML operation is performed. So use indexes
only on columns which are used to search the table frequently.
2) Is is not required to
create indexes on table which have less data.
In oracle database you can
define up to sixteen (16) columns in an INDEX.
DCL commands are used to
enforce database security in a multiple user database environment. Two types of
DCL commands are GRANT and REVOTE. Only Database Administrator's or owner's of
the database object can provide/remove privileges on a databse object.
SQL GRANT Command
SQL GRANT is a command used
to provide access or privileges on the database objects to the users.
The Syntax for the GRANT
command is:
GRANT privilege_name
ON object_name
TO {user_name |PUBLIC
|role_name}
[WITH GRANT OPTION];
•privilege_name is the
access right or privilege granted to the user. Some of the access rights are
ALL, EXECUTE, and SELECT.
•object_name is the name of
an database object like TABLE, VIEW, STORED PROC and SEQUENCE.
•user_name is the name of
the user to whom an access right is being granted.
•user_name is the name of
the user to whom an access right is being granted.
•PUBLIC is used to grant
access rights to all users.
•ROLES are a set of
privileges grouped together.
ITH GRANT OPTION - allows a
user to grant access rights to other users.
For Eample: GRANT SELECT ON
employee TO user1;This command grants a SELECT permission on employee table to
user1.You should use the WITH GRANT option carefully because for example if you
GRANT SELECT privilege on employee table to user1 using the WITH GRANT option,
then user1 can GRANT SELECT privilege on employee table to another user, such
as user2 etc. Later, if you REVOKE the SELECT privilege on employee from user1,
still user2 will have SELECT privilege on employee table.
SQL REVOKE Command:
The REVOKE command removes
user access rights or privileges to the database objects.
The Syntax for the REVOKE
command is:
REVOKE privilege_name
ON object_name
FROM {user_name |PUBLIC
|role_name}
For Eample: REVOKE SELECT ON
employee FROM user1;This commmand will REVOKE a SELECT privilege on employee
table from user1.When you REVOKE SELECT privilege on a table from a user, the
user will not be able to SELECT data from that table anymore. However, if the
user has received SELECT privileges on that table from more than one users,
he/she can SELECT from that table until everyone who granted the permission
revokes it. You cannot REVOKE privileges if they were not initially granted by
you.
Privileges and Roles:
Privileges: Privileges
defines the access rights provided to a user on a database object. There are
two types of privileges.
1) System privileges - This
allows the user to CREATE, ALTER, or DROP database objects.
2) Object privileges - This
allows the user to EXECUTE, SELECT, INSERT, UPDATE, or DELETE data from
database objects to which the privileges apply.
Few CREATE system privileges
are listed below:
System Privileges
Description
CREATE object allows users
to create the specified object in their own schema.
CREATE ANY object allows
users to create the specified object in any schema.
The above rules also apply
for ALTER and DROP system privileges.
Few of the object privileges
are listed below:
vileges Description
INSERT allows users to insert rows into a table.
SELECT allows users to select data from a database
object.
UPDATE allows user to update data in a table.
ECUTE allows user to execute a stored procedure or
a function.
Roles: Roles are a
collection of privileges or access rights. When there are many users in a
database it becomes difficult to grant or revoke privileges to users.
Therefore, if you define roles, you can grant or revoke privileges to users,
thereby automatically granting or revoking privileges. You can either create
Roles or use the system roles pre-defined by oracle.
Some of the privileges
granted to the system roles are as given below:
System Role Privileges Granted to the Role
CONNECT CREATE TABLE, CREATE VIEW, CREATE SYNONYM,
CREATE SEQUENCE, CREATE SESSION etc.
RESOURCE CREATE PROCEDURE, CREATE SEQUENCE, CREATE
TABLE, CREATE TRIGGER etc. The primary usage of the RESOURCE role is to
restrict access to database objects.
DBA ALL SYSTEM PRIVILEGES
Creating Roles:
The Syntax to create a role
is:
CREATE ROLE role_name
[IDENTIFIED BY password];
example: To create a role
called "developer" with password as "pwd",the code will be
as follows
CREATE ROLE testing
[IDENTIFIED BY pwd];
It's easier to GRANT or
REVOKE privileges to the users through a role rather than assigning a privilege
direclty to every user. If a role is identified by a password, then, when you
GRANT or REVOKE privileges to the role, you definetely have to identify it with
the password.
We can GRANT or REVOKE
privilege to a role as below.
For example: To grant CREATE
TABLE privilege to a user by creating a testing role:
First, create a testing Role
CREATE ROLE testing
Second, grant a CREATE TABLE
privilege to the ROLE testing. You can add more privileges to the ROLE.
GRANT CREATE TABLE TO
testing;
rd, grant the role to a
user.
O user1;
To revoke a CREATE TABLE
privilege from testing ROLE, you can write:
REVOKE CREATE TABLE FROM
testing;
The Syntax to drop a role
from the database is as below:
DROP ROLE role_name;
For example: To drop a role
called developer, you can write:
DROP ROLE testing;
Oracle Built in Functions
There are two types of
functions in Oracle.
1) Single Row Functions:
Single row or Scalar functions return a value for every row that is processed
in a query.
2) Group Functions: These
functions group the rows of data based on the values returned by the query.
This is discussed in SQL GROUP Functions. The group functions are used to
calculate aggregate values like total or average, which return just one total
or one average value after processing a group of rows.
There are four types of
single row functions. They are:
1) Numeric Functions: These
are functions that accept numeric input and return numeric values.
2) Character or Text
Functions: These are functions that accept character input and can return
both number values.
3) Date Functions: These are
functions that take values that are of datatype DATE as input and return values
of datatype DATE, except for the MONTHS_BETWEEN function, which returns a
number.
4) Conversion Functions:
These are functions that help us to convert a value in one form to another
form. For Example: a null value into an actual value, or a value from one
datatype to another datatype like NVL, TO_CHAR, TO_NUMBER, TO_DATE etc.
You can combine more than
one function together in an expression. This is known as nesting of functions.
What is a DUAL Table in
Oracle?
This is a single row and
single column dummy table provided by oracle. This is used to perform
mathematical calculations without using a table.
Select * from DUAL
Output:
DUMMY
-------
X
Select 777 * 888 from Dual
Output:
777 * 888
---------
689976
1) Numeric Functions:
Numeric functions are used
to perform operations on numbers. They accept numeric values as input and
return numeric values as output. Few of the Numeric functions are:
Function Name Return Value
ABS (x) Absolute value of
the number 'x'
CEIL (x) Integer value that
is Greater than or equal to the number 'x'
FLOOR (x) Integer value that
is Less than or equal to the number 'x'
TRUNC (x, y) Truncates value
of number 'x' up to 'y' decimal places
ROUND (x, y) Rounded off
value of the number 'x' up to the number 'y' decimal places
The following examples
explains the usage of the above numeric functions
Function Name Examples
Return Value
ABS (x) ABS (1)
ABS (-1) 1
-1
CEIL (x) CEIL (2.83)
CEIL (2.49)
CEIL (-1.6) 3
3
-1
FLOOR (x) FLOOR (2.83)
FLOOR (2.49)
FLOOR (-1.6) 2
2
-2
TRUNC (x, y) ROUND (125.456,
1)
ROUND (125.456, 0)
ROUND (124.456, -1) 125.4
125
120
ROUND (x, y) TRUNC (140.234,
2)
TRUNC (-54, 1)
TRUNC (5.7)
TRUNC (142, -1) 140.23
54
5
140
These functions can be used
on database columns.
For Example: Let's consider
the product table used in sql joins. We can use ROUND to round off the unit_price
to the nearest integer, if any product has prices in fraction.
SELECT ROUND (unit_price)
FROM product;
2) Character or Text
Functions:
Character or text functions
are used to manipulate text strings. They accept strings or characters as input
and can return both character and number values as output.
Few of the character or text
functions are as given below:
Function Name Return Value
LOWER (string_value) All the
letters in 'string_value' is converted to lowercase.
UPPER (string_value) All the
letters in 'string_value' is converted to uppercase.
INITCAP (string_value) All the letters in 'string_value' is
converted to mixed case.
LTRIM (string_value,
trim_text) All occurrences of
'trim_text' is removed from the left of 'string_value'.
RTRIM (string_value,
trim_text) All occurrences of
'trim_text' is removed from the right of 'string_value' .
TRIM (trim_text FROM
string_value) All occurrences of
'trim_text' from the left and right of 'string_value' , 'trim_text' can also be
only one character long .
SUBSTR (string_value, m,
n) Returns 'n' number of characters from
'string_value' starting from the 'm' position.
LENGTH (string_value) Number of characters in 'string_value' in
returned.
LPAD (string_value, n,
pad_value) Returns 'string_value'
left-padded with 'pad_value' . The length of the whole string will be of 'n'
characters.
RPAD (string_value, n,
pad_value) Returns 'string_value'
right-padded with 'pad_value' . The length of the whole string will be of 'n'
characters.
For Example, we can use the
above UPPER() text function with the column value as follows.
SELECT UPPER (product_name)
FROM product;
The following examples
explains the usage of the above character or text functions
Function Name Examples
Return Value
LOWER(string_value) LOWER('Good Morning') good morning
UPPER(string_value)
UPPER('Good Morning') GOOD MORNING
INITCAP(string_value) INITCAP('GOOD MORNING') Good Morning
LTRIM(string_value,
trim_text) LTRIM ('Good Morning', 'Good) Morning
RTRIM (string_value,
trim_text) RTRIM ('Good Morning', ' Morning') Good
TRIM (trim_text FROM
string_value) TRIM ('o' FROM 'Good
Morning') Gd Mrning
SUBSTR (string_value, m,
n) SUBSTR ('Good Morning', 6, 7) Morning
LENGTH (string_value) LENGTH ('Good Morning') 12
LPAD (string_value, n,
pad_value) LPAD ('Good', 6, '*') **Good
RPAD (string_value, n,
pad_value) RPAD ('Good', 6, '*') Good**
3) Date Functions:
These are functions that
take values that are of datatype DATE as input and return values of datatypes
DATE, except for the MONTHS_BETWEEN function, which returns a number as output.
Few date functions are as
given below.
Function Name Return Value
ADD_MONTHS (date, n) Returns
a date value after adding 'n' months to the date 'x'.
MONTHS_BETWEEN (x1, x2)
Returns the number of months between dates x1 and x2.
ROUND (x, date_format)
Returns the date 'x' rounded off to the nearest century, year, month, date,
hour, minute, or second as specified by the 'date_format'.
TRUNC (x, date_format)
Returns the date 'x' lesser than or equal to the nearest century, year, month,
date, hour, minute, or second as specified by the 'date_format'.
NEXT_DAY (x, week_day)
Returns the next date of the 'week_day' on or after the date 'x' occurs.
LAST_DAY (x) It is used to determine the number of days
remaining in a month from the date 'x' specified.
SYSDATE Returns the systems
current date and time.
NEW_TIME (x, zone1, zone2)
Returns the date and time in zone2 if date 'x' represents the time in zone1.
The below table provides the
examples for the above functions
Function Name Examples
Return Value
ADD_MONTHS ( ) ADD_MONTHS
('16-Sep-81', 3) 16-Dec-81
MONTHS_BETWEEN( )
MONTHS_BETWEEN ('16-Sep-81', '16-Dec-81')
3
NEXT_DAY( ) NEXT_DAY ('01-Jun-08', 'Wednesday') 04-JUN-08
LAST_DAY( ) LAST_DAY
('01-Jun-08') 30-Jun-08
NEW_TIME( ) NEW_TIME
('01-Jun-08', 'IST', 'EST') 31-May-08
4) Conversion Functions:
These are functions that
help us to convert a value in one form to another form. For Ex: a null value
into an actual value, or a value from one datatype to another datatype like
NVL, TO_CHAR, TO_NUMBER, TO_DATE.
Few of the conversion
functions available in oracle are:
Function Name Return Value
TO_CHAR (x [,y]) Converts
Numeric and Date values to a character string value. It cannot be used for
calculations since it is a string value.
TO_DATE (x [, date_format])
Converts a valid Numeric and Character values to a Date value. Date is
formatted to the format specified by 'date_format'.
NVL (x, y) If 'x' is NULL,
replace it with 'y'. 'x' and 'y' must be of the same datatype.
DECODE (a, b, c, d, e,
default_value) Checks the value of 'a',
if a = b, then returns 'c'. If a = d, then returns 'e'. Else, returns
default_value.
The below table provides the
examples for the above functions
Function Name Examples
Return Value
TO_CHAR () TO_CHAR (3000, '$9999')
TO_CHAR (SYSDATE, 'Day,
Month YYYY') $3000
Monday, June 2008
TO_DATE () TO_DATE ('01-Jun-08') 01-Jun-08
NVL () NVL (null, 1)
1
SQL Tuning or SQL
Optimization
Sql Statements are used to
retrieve data from the database. We can get same results by writing different
sql queries. But use of the best query is important when performance is
considered. So you need to sql query tuning based on the requirement. Here is
the list of queries which we use reqularly and how these sql queries can be
optimized for better performance.
SQL Tuning/SQL Optimization Techniques:
1) The sql query becomes
faster if you use the actual columns names in SELECT statement instead of than
'*'.
For Example: Write the query
as
SELECT id, first_name,
last_name, age, subject FROM student_details;
Instead of:
SELECT * FROM
student_details;
2) HAVING clause is used to
filter the rows after all the rows are selected. It is just like a filter. Do
not use HAVING clause for any other purposes.
For Example: Write the query
as
SELECT subject, count(subject)
FROM student_details
WHERE subject != 'Science'
AND subject != 'Maths'
GROUP BY subject;
Instead of:
SELECT subject,
count(subject)
FROM student_details
GROUP BY subject
HAVING subject!= 'Vancouver'
AND subject!= 'Toronto';
3) Sometimes you may have
more than one subqueries in your main query. Try to minimize the number of
subquery block in your query.
For Example: Write the query
as
SELECT name
FROM employee
WHERE (salary, age ) =
(SELECT MAX (salary), MAX (age)
FROM employee_details)
AND dept = 'Electronics';
Instead of:
SELECT name
FROM employee
WHERE salary = (SELECT
MAX(salary) FROM employee_details)
AND age = (SELECT MAX(age)
FROM employee_details)
AND emp_dept =
'Electronics';
4) Use operator EXISTS, IN
and table joins appropriately in your query.
a) Usually IN has the
slowest performance.
b) IN is efficient when most
of the filter criteria is in the sub-query.
c) EXISTS is efficient when
most of the filter criteria is in the main query.
For Example: Write the query
as
Select * from product p
where EXISTS (select * from
order_items o
where o.product_id =
p.product_id)
Instead of:
Select * from product p
where product_id IN
(select product_id from
order_items
5) Use EXISTS instead of
DISTINCT when using joins which involves tables having one-to-many
relationship.
For Example: Write the query
as
SELECT d.dept_id, d.dept
FROM dept d
WHERE EXISTS ( SELECT 'X'
FROM employee e WHERE e.dept = d.dept);
Instead of:
SELECT DISTINCT d.dept_id,
d.dept
FROM dept d,employee e
WHERE e.dept = e.dept;
6) Try to use UNION ALL in
place of UNION.
For Example: Write the query
as
SELECT id, first_name
FROM student_details_class10
UNION ALL
SELECT id, first_name
FROM sports_team;
Instead of:
SELECT id, first_name,
subject
FROM student_details_class10
UNION
SELECT id, first_name
FROM sports_team;
7) Be careful while using
conditions in WHERE clause.
For Example: Write the query
as
SELECT id, first_name, age
FROM student_details WHERE age > 10;
Instead of:
SELECT id, first_name, age
FROM student_details WHERE age != 10;
Write the query as
SELECT id, first_name, age
FROM student_details
WHERE first_name LIKE
'Chan%';
Instead of:
SELECT id, first_name, age
FROM student_details
WHERE SUBSTR(first_name,1,3)
= 'Cha';
Write the query as
SELECT id, first_name, age
FROM student_details
WHERE first_name LIKE NVL (
:name, '%');
Instead of:
SELECT id, first_name, age
FROM student_details
WHERE first_name = NVL (
:name, first_name);
Write the query as
SELECT product_id,
product_name
FROM product
WHERE unit_price BETWEEN
MAX(unit_price) and MIN(unit_price)
Instead of:
SELECT product_id,
product_name
FROM product
WHERE unit_price >= MAX(unit_price)
and unit_price <=
MIN(unit_price)
Write the query as
SELECT id, name, salary
FROM employee
WHERE dept = 'Electronics'
AND location = 'Bangalore';
Instead of:
SELECT id, name, salary
FROM employee
WHERE dept || location= 'ElectronicsBangalore';
Use non-column expression on
one side of the query because it will be processed earlier.
Write the query as
SELECT id, name, salary
FROM employee
WHERE salary < 25000;
Instead of:
SELECT id, name, salary
FROM employee
WHERE salary + 10000 <
35000;
Write the query as
SELECT id, first_name, age
FROM student_details
WHERE age > 10;
Instead of:
SELECT id, first_name, age
FROM student_details
WHERE age NOT = 10;
8) Use DECODE to avoid the
scanning of same rows or joining the same table repetitively. DECODE can also
be made used in place of GROUP BY or ORDER BY clause.
For Example: Write the query
as
SELECT id FROM employee
WHERE name LIKE 'Ramesh%'
and location = 'Bangalore';
Instead of:
SELECT DECODE(location,'Bangalore',id,NULL)
id FROM employee
WHERE name LIKE 'Ramesh%';
9) To store large binary
objects, first place them in the file system and add the file path in the
database.
10) To write queries which
provide efficient performance follow the general SQL standard rules.
a) Use single case for all
SQL verbs
b) Begin all SQL verbs on a
new line
c) Separate all words with a
single space
d) Right or left aligning
verbs within the initial SQL verb
Question:
Find the Nth maximum salary from Table.
SELECT *
FROM Employee_Test Emp1
WHERE ( n ) = (
SELECT COUNT( DISTINCT ( Emp2.Employee_Salary ) )
FROM Employee_Test Emp2
WHERE Emp2.Employee_Salary >= Emp1.Employee_Salary
)
The inner query executes every time; a row
from outer query is processed. Inner query essentially does not do any very
secret job, it only return the count of distinct salaries, which are
higher than the currently processing row’s salary column. Anytime, it find that
salary column’s value of current row from outer query, is equal to count of
higher salaries from inner query, it returns the result.
Delete Duplicate values from a table:
DELETE FROM
table_name A WHERE ROWID > ( SELECT min(rowid) FROM table_name B WHERE A.key_values
= B.key_values);
Method -1
Delete all rowids that is BIGGER than the SMALLEST
rowid value (for a given key).
delete from TableName t1 where rowID >
(Select min(RowID) from TableName T2 where t1.column1 =
T2.column1)
Method - 2
This query removes rows based on the column names
specified in the GROUP BY clause. If you specify only one column name it will
remove all duplicate records for that column. If you want to delete exact
replica's of the same row - use all the column names in the GROUP BY.
DELETE FROM dbo.ATTENDANCE WHERE AUTOID NOT IN (SELECT
MIN(AUTOID)
FROM
dbo.ATTENDANCE GROUP BY EMPLOYEE_ID,ATTENDANCE_DATE)
Method 3:
SQL> create table table_name2 as select distinct *
from table_name1;
SQL> drop table table_name1;
SQL> rename table_name2 to table_name1;
