WHERE Clause

Filter rows — get only the data you actually need

Beginner 14 minWHEREfilteringconditions

WHERE: Filtering Rows

WHERE filters which rows appear in your results. Think of SELECT as choosing columns (vertical slice) and WHERE as choosing rows (horizontal slice). Together, they let you extract the exact rectangle of data you need. Here's how it works:

Condition-based filtering — WHERE evaluates a condition for every row in the table. If the condition is true, the row is included. If it's false or NULL, the row is excluded from the results entirely.

Runs before SELECT — WHERE executes before column selection, so you can filter on columns you don't display. WHERE price > 50 works even when price isn't in your SELECT list at all.

Comparison operators — Use =, != (or <>), <, >, <=, >= to build conditions. These work with numbers, text, and dates depending on the column type.

Text needs single quotes — WHERE city = 'Mumbai' requires single quotes around text values. Numbers don't need any quotes. Using double quotes will cause an error in most databases.

Knowledge Canvas

How WHERE Works

Filter rows before they reach your output

WHERE evaluates each row and keeps only TRUE ones
Runs AFTER FROM but BEFORE SELECT
Multiple conditions combine with AND / OR
Cannot use column aliases — use the original expression

Common Traps

WHERE pitfalls that waste hours

= NULL never works — use IS NULL instead
String = comparisons: PostgreSQL, Oracle, and SQLite are case-sensitive by default; MySQL and SQL Server are typically case-insensitive
WHERE runs before GROUP BY — can't filter aggregates here
Using OR without parentheses changes logic unexpectedly

WHERE vs HAVING

Different filters for different stages

WHERE: filters individual rows

HAVING: filters groups after aggregation

Runs before GROUP BY

Runs after GROUP BY

Cannot use aggregate functions

Designed for aggregate conditions

WHERE price > 100

HAVING COUNT(*) > 5

Useful Patterns

Common WHERE techniques

Range: WHERE price BETWEEN 10 AND 50
List: WHERE country IN ('IN', 'US', 'UK')
Pattern: WHERE email LIKE '%@gmail.com'
Null check: WHERE phone IS NOT NULL

Performance Tips

Keep WHERE clauses fast

Put the most selective condition first
Avoid functions on indexed columns: WHERE YEAR(date) is slow
Use = over LIKE when exact match is enough
Index columns you frequently filter on

Syntax

Syntax Template

1SELECT

2 columns

3FROM table

4WHERE condition;

6-- Comparison operators

7-- WHERE price > 50

8-- WHERE status = 'delivered'

9-- WHERE stock != 0

10-- WHERE joined_date >= '2024-01-01'

=Equals (use single quotes for text values)

!= or <>Not equal

< > <= >=Less/greater than, with or without equals

Single quotesRequired for text: WHERE city = 'Mumbai' (not double quotes)

Sample Data

products

10 rows

id	name	category	price	stock
P101	Wireless Mouse	Electronics	24.99	150
P102	Office Desk	Furniture	189	25
P103	LED Desk Lamp	Furniture	45.5	80
P104	USB-C Cable	Electronics	12.99	500
P105	Bluetooth Speaker	Electronics	79	60
P106	Office Chair	Furniture	95	15
P107	Coffee Maker	Appliances	89.99	40
P108	Notebook Set	Stationery	8.5	200
P109	Wireless Headphones	Electronics	159	30
P110	Standing Desk	Furniture	425	8

Worked Example

Find all products that cost more than 50.

SQL

1SELECT

2 name,

3 price

4FROM products

5WHERE price > 50;

The database scans every row in the products table and checks if price > 50. Five products pass the filter: Standing Desk (449), Monitor 27inch (329), Office Chair (299), Mechanical Keyboard (89.99), and Webcam HD (59.99). Products priced at 50 or below — like Wireless Mouse (29.99) and Notebook Set (12.50) — are excluded.

Output

5 rows

name	price
Office Desk	189
Bluetooth Speaker	79
Office Chair	95
Coffee Maker	89.99
Wireless Headphones	159

Common Mistakes

❌

Forgetting quotes around text values

SQL

1SELECT

2 *

3FROM products

4WHERE category = Electronics;

Without quotes, SQL thinks Electronics is a column name, not a text value. This causes a "no such column" error.

Always wrap text in single quotes: WHERE category = 'Electronics'

❌

Using = for NULL comparisons

SQL

1SELECT

2 *

3FROM reviews

4WHERE comment = NULL;

NULL is not a value — it means "unknown." You cannot compare it with =. This returns zero rows even if NULLs exist.

Use IS NULL or IS NOT NULL instead (covered in a later topic).

Key Concepts

1WHERE runs before SELECT — filter on columns you don't display

2Single quotes for text: WHERE city = 'Mumbai' (not double quotes)

3NULL comparisons need IS NULL — not = or !=

4Without WHERE, every query returns the entire table

Pro Tip

Without WHERE, every query returns the full table. WHERE is how you ask specific questions: "Which orders are pending?", "What products cost over 100?"

When to Use

Finding specific records, filtering by date range, showing only in-stock products, retrieving data for a single user ID.

Challenge

Solve the problem below

Find all products in the 'Furniture' category that cost less than 100. Show the name and price.

Your Query

SELECT Statement

AND Operator