SQL WHERE Clause

What is the WHERE Clause?

The WHERE clause filters rows returned by a SELECT statement. Only rows that satisfy the specified condition appear in the result set.

Without WHERE, a query returns every row in the table. Adding WHERE lets you narrow the results to exactly the data you need.

Syntax

SELECT column1, column2
FROM table_name
WHERE condition;

Conditions use comparison operators such as =, !=, <, >, <=, and >=. You can combine multiple conditions with AND, OR, and NOT.

When to Use

• Fetching a specific record by its ID
• Filtering products above a certain price
• Finding students in a particular grade
• Limiting results to a date range

Key Points

1. Evaluated Per Row — The database checks the WHERE condition against every row and only includes rows where the condition is true.
2. NULL Handling — Comparisons with NULL using = return unknown, not true or false. Use IS NULL instead.
3. Index Usage — Filtering on indexed columns makes queries faster because the database can skip irrelevant rows.
4. Execution Order — WHERE is processed before SELECT, so you cannot reference column aliases defined in the SELECT list.
5. String Comparisons — Wrap string literals in single quotes: WHERE first_name = 'Alice'.

The WHERE clause is essential for every real-world query. Combine it with other clauses like ORDER BY and LIMIT to build powerful, targeted queries.