SQL Tutorial: Where Clause

Where---à where clause is used to filter records

Explanation:

Where clause restricts records that is it helps as to apply conditions on SQL statements to get our desired specific records rather than getting all generalized record

Restrict/Limit the returned rows by Where clause

 Three Elements of WHERE Clause:

1. Column Name
2. Comparison Operator
3. Column Name, Constant, List of Values

Example: When you write SELECT SQL statement with * to get records of employees from the employee table you get data from all columns and all rows present in the table of database but the point is sometimes you do not need to know the data of all employees so that makes extra data fetch by SELECT * useless and obviously fetching all data from the table of the database will make database busy from more time period and the database will make the system slow. To eliminate this issue we define “where” clause just after the FROM clause to get specific results without over burden the database.

Consider the example of Employee table. Let’s there be 3 rows in the table.

emp_id	emp_firstname	emp_secondname	emp_address
001	John	smith	Abc
002	Andrew	douglas	Def
003	Jack	Jackson	ghi

 Figure: 1.1

If you execute SELECT statement

Select *

From Employees;

It will display all the 3 records of 3 employees that is the entire table as given in figure 1.1

Select *

From employee

Where emp_id=001;

Now only the data related to employee with emp_id 001 is going to be displayed

emp_id	emp_firstname	emp_secondname	emp_address
001	John	smith	Abc