A subquery is a query nested in another query.

Title Description
Scalar Subqueries Subquery returning a single value
Row Subqueries Subquery returning a row
Subqueries and ALL Return true if the comparison returns true for each row, or the subquery returns no rows.
Subqueries and ANY Return true if the comparison returns true for at least one row returned by the subquery.
Subqueries and EXISTS Returns true if the subquery returns any rows.
Subqueries in a FROM Clause Subqueries are more commonly placed in a WHERE clause, but can also form part of the FROM clause
Subquery Optimizations Articles about subquery optimizations in MariaDB.
Subqueries and JOINs Rewriting subqueries as JOINs, and using subqueries instead of JOINs
Subquery Limitations There are a number of limitations regarding subqueries.
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