Subqueries

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Subqueries are a method of running two or more queries within another single query to access information stored in multiple database tables. Subqueries are supported in most professional-grade RDBMS-es, including MySQL >= 4.1, MSSQL, Oracle, Postgres, and DB2.

Example:

SELECT X.*, ( SELECT username FROM users WHERE id=X.user_id ) FROM data_table X