SQL Where Clause

The WHERE Clause is used when you want to retrieve specific information from a table excluding other irrelevant data.

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:

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; 

NOTE:Comparison Operators and Logical Operators are used in WHERE Clause.
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.


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 be used in WHERE Clause.