Subqueries and CTEs — Organizing Complex Queries
Learn to write subqueries and Common Table Expressions to break complex SQL into readable, maintainable pieces.
At some point, a single SELECT statement isn't enough. You need the result of one query to feed into another. Maybe you want to find customers whose order total exceeds the average. Maybe you need to rank products within each category. Maybe your query is getting so long that nobody (including future you) can follow it.
Subqueries and Common Table Expressions (CTEs) are how you handle this. They let you compose queries from smaller, logical building blocks.
Subqueries: A Query Inside a Query
A subquery is exactly what it sounds like — a SELECT statement nested inside another SQL statement. The inner query runs first, and its result is used by the outer query.
Subqueries in WHERE Clauses
The most common use. Find rows that match a condition based on another query's result:
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