SqlDatabaseBeginner

WHERE Clause

Filters rows returned by a query using a boolean condition.

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SELECT columns FROM table_name WHERE condition;
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What it does

Overview

Filters rows returned by a query using a boolean condition.

The WHERE clause acts as a logical filter that evaluates a predicate for every row provided by the FROM clause. It uses three-valued logic, where comparisons result in TRUE, FALSE, or UNKNOWN (typically due to NULL values). For optimal performance, predicates should be 'SARGable' (Search ARgumentable), meaning they should avoid applying functions to indexed columns—for instance, using `olumn >= '2023-01-01'` is significantly faster than `YEAR(olumn) = 2023` because the latter prevents the engine from utilizing B-tree indexes. Unlike the HAVING clause, which filters post-aggregation, the WHERE clause filters individual records before any grouping or summarization occurs. When handling date comparisons, it is standard practice to use ISO 8601 strings (YYYY-MM-DD) or explicit casting to ensure deterministic behavior across different database locales and engines.

Quick reference

Syntax

SELECT columns FROM table_name WHERE condition;

Inputs

Parameters

conditionBoolean expression to filter rows (e.g., age >= 18).

See it in practice

Examples

1

Basic Conditional Comparison

SELECT name, price FROM products WHERE price >= 150.00;
Output:
A result set containing only products with a price of 150 or higher.

Uses a standard comparison operator to filter numeric data.

2

Compound Predicates and Precedence

SELECT * FROM orders WHERE status = 'Processed' AND (priority = 'High' OR total_amount > 1000);
Output:
Processed orders that are either marked as high priority or have a large total.

Utilizes parentheses to force the OR condition to evaluate before the AND condition, preventing logic errors in complex filters.

3

Pattern Matching with Wildcards

SELECT username FROM users WHERE email LIKE '%@company.com';
Output:
A list of usernames associated with the specific company domain.

Employs the LIKE operator and the '%' wildcard to perform partial string matching on column data.

Debug faster

Common Errors

1

LogicError

Cause: Attempting to evaluate NULL values using the equality operator (=). In SQL, NULL represents an unknown value, and any direct comparison with it (except via IS) returns UNKNOWN.

Fix: Use the IS NULL or IS NOT NULL operators instead of the equals sign.

SELECT * FROM employees WHERE supervisor_id = NULL; -- Incorrect: returns 0 rows
2

ReferenceError

Cause: Referencing a column alias defined in the SELECT clause within the WHERE clause. SQL evaluates the WHERE clause before the SELECT projection is processed.

Fix: Repeat the original column expression in the WHERE clause or use a Subquery/CTE.

SELECT price * 0.1 AS tax FROM sales WHERE tax > 5; -- Incorrect: 'tax' is not yet defined

Runtime support

Compatibility

All SQL databasesSQL-92 (Standard)

Core filtering feature

Source: MDN Web Docs

Common questions

Frequently Asked Questions

Filters rows returned by a query using a boolean condition.

condition: Boolean expression to filter rows (e.g., age >= 18).

LogicError: Use the IS NULL or IS NOT NULL operators instead of the equals sign. ReferenceError: Repeat the original column expression in the WHERE clause or use a Subquery/CTE.