What is a subquery and when would you use it?

Parthiban Kannan

Updated on January 8, 2026

< 1 min read

A subquery is a query written inside another SQL query.
It is used when results from one query are needed as input for another.
Subqueries help apply dynamic filters or calculations.
In my project, we used a subquery to find customers with above-average sales.
It kept the logic clear without creating temporary tables.
Subqueries are common in WHERE, SELECT, or FROM clauses.
They are useful but can impact performance if not optimized.