Why this chapter matters
Some business questions are naturally phrased as nested logic, such as find customers above their segment average.
What you will learn
- Write scalar, table, and EXISTS subqueries for practical filters.
- Use correlated subqueries only when they improve clarity.
- Rewrite subqueries into joins when performance or readability benefits.
Lessons in this chapter
- Subquery formsDifferentiate scalar, IN, and EXISTS patterns in PostgreSQL. Read the full guide →
- Correlated logicApply row-by-row correlation carefully and verify intent with sample outputs.
- IN versus EXISTSPick the form that expresses intent clearly and performs well for dataset size.
- Refactoring nested queriesConvert deep nesting into cleaner shapes when maintenance cost is high.
Study task
Find products in PostgreSQL whose price is above the average price within their own category.
Chapter checkpoint
What does EXISTS check in a PostgreSQL subquery?
EXISTS returns true when the subquery yields at least one row.