8.1 The WHERE Clause

Level: 8 | Version: v1.1 | Author: Meptrasoft

Overview

The WHERE clause is used to filter rows based on a condition.

For example:

SELECT *
FROM employees
WHERE dept_id = 10;

Only employees whose dept_id is 10 are returned.

Common filtering operators and clauses include:

WHERE
IN / NOT IN
BETWEEN
IS / IS NOT
LIKE / NOT LIKE

These will be covered one by one in this module.

WHERE → Keep only the rows that match a condition.

SQL WHERE Clause — Filtering Rows based on conditions — Figure 1: The WHERE clause filters table rows and returns only the records that satisfy a condition.

Practice Table

This topic uses the employees table:

emp_id	emp_name	dept_id	salary	phone_no
1	John Smith	10	45000	9876543210
2	Emma Brown	20	60000	NULL
3	Samuel Reed	10	32000	9123456780
4	Anderson	30	55000	NULL
5	Sarah Wilson	20	48000	9988776655
6	Tom Watson	10	51000	9090909090

Basic WHERE

Filter employees from department 10:

SELECT *
FROM employees
WHERE dept_id = 10;

Result:

emp_id	emp_name	dept_id	salary
1	John Smith	10	45000
3	Samuel Reed	10	32000
6	Tom Watson	10	51000

WHERE with a Numeric Condition

You can also filter numeric values.

SELECT *
FROM employees
WHERE salary > 50000;

Result:

emp_id	emp_name	salary
2	Emma Brown	60000
4	Anderson	55000
6	Tom Watson	51000

Common comparison operators include:

=    → Equal
<>   → Not equal
>    → Greater than
<    → Less than
>=   → Greater than or equal
<=   → Less than or equal

WHERE with Text

Text values are usually written inside single quotes:

SELECT *
FROM employees
WHERE emp_name = 'John Smith';

This returns only the row where the name matches exactly.

WHERE with Multiple Conditions

Conditions can be combined using AND and OR.

AND

Both conditions must be true.

SELECT *
FROM employees
WHERE dept_id = 10
  AND salary > 40000;

OR

At least one condition must be true.

SELECT *
FROM employees
WHERE dept_id = 10
   OR dept_id = 20;

AND → Both conditions
OR → At least one condition

Important: NULL Values

NULL means a value is unknown or missing.

Do not use:

WHERE phone_no = NULL

Instead, use:

SELECT *
FROM employees
WHERE phone_no IS NULL;

And for non-NULL values:

SELECT *
FROM employees
WHERE phone_no IS NOT NULL;

IS and IS NOT will be covered in detail later.

WHERE vs SELECT

A common beginner confusion:

SELECT → Which columns?
WHERE  → Which rows?

For example:

SELECT emp_name, salary
FROM employees
WHERE dept_id = 10;

Here:

SELECT chooses emp_name and salary.
WHERE keeps only employees from department 10.

Common Beginner Mistakes

✗ Mistake	✓ Correct Understanding
`WHERE` chooses columns	`WHERE` filters rows
Text values do not need quotes	String literals are normally written using single quotes
`= NULL` checks for NULL	Use `IS NULL` or `IS NOT NULL`
`AND` means either condition can be true	Both conditions must be true
`OR` means both conditions must be true	At least one condition must be true
`WHERE` permanently removes rows	It only filters the query result

Placement Quick Points

WHERE
→ FILTER ROWS

=
→ EQUAL

>
→ GREATER THAN

AND
→ BOTH CONDITIONS

OR
→ AT LEAST ONE CONDITION

IS NULL
→ CHECK FOR NULL

WHERE filters rows based on conditions.
Comparison operators can be used inside WHERE.
Text values are normally enclosed in single quotes.
AND requires all combined conditions to be true.
OR requires at least one condition to be true.
Use IS NULL and IS NOT NULL for NULL checks.
WHERE does not change the stored table.

Interview Questions

What is the purpose of WHERE?

WHERE filters rows based on a specified condition.

What is the difference between SELECT and WHERE?

SELECT → Chooses columns
WHERE  → Filters rows

How do you check for NULL?

WHERE phone_no IS NULL;

What is the difference between AND and OR?

AND requires all conditions to be true, while OR requires at least one condition to be true.

Does WHERE modify the data?

No. It only filters the rows returned by the query.

Can WHERE be used with text and numbers?

Yes.

WHERE department = 'IT'

and:

WHERE salary > 50000

Practice & Hands-On Exercises

Using the employees table:

Find employees from department 10:

SELECT * FROM employees WHERE dept_id = 10;

Find employees with salary greater than 40000:

SELECT * FROM employees WHERE salary > 40000;

Find the employee named John Smith:

SELECT * FROM employees WHERE emp_name = 'John Smith';

Find employees in department 10 with salary greater than 40000:

SELECT * FROM employees WHERE dept_id = 10 AND salary > 40000;

Find employees in department 10 or 20:

SELECT * FROM employees WHERE dept_id = 10 OR dept_id = 20;

Find employees whose phone_no is NULL:

SELECT * FROM employees WHERE phone_no IS NULL;

Explain the difference between SELECT and WHERE.
Explain the difference between AND and OR.

💡 Tip: Test your queries using the in-browser interactive runner above.

Key Takeaway

SELECT → CHOOSE COLUMNS
WHERE  → FILTER ROWS

The WHERE clause filters rows by applying conditions, allowing SQL to return only the data that matches the requirement.

End of The WHERE Clause