Subquery

Advanced

A query nested inside another query. Can appear in SELECT, FROM, WHERE, or HAVING.

Syntax

SELECT col FROM table WHERE col IN (SELECT ...)

Example

SELECT * FROM products
WHERE price > (
  SELECT AVG(price) FROM products
);

About SQL Subquery

The Subquery keyword belongs to the Advanced category of SQL statements. A query nested inside another query. Can appear in SELECT, FROM, WHERE, or HAVING. Understanding this command is essential for any developer working with relational databases like PostgreSQL, MySQL, SQLite, or SQL Server.

SQL (Structured Query Language) is the standard language for managing and querying relational databases. The Subquery statement is supported across all major database systems, though specific syntax may vary slightly between PostgreSQL, MySQL, MariaDB, Oracle, SQL Server, and SQLite. Always consult your database's documentation for vendor-specific features and limitations.

Best practices for using Subquery: always test queries on a development database before running them in production, use parameterized queries to prevent SQL injection, and leverage EXPLAIN to understand query performance. For complex queries, consider using CTEs (Common Table Expressions) to improve readability and maintainability.

Related SQL Keywords

EXISTSINANYALL