beginner~4h
Subqueries, EXISTS & Correlated Subqueries
Subqueries are SELECT statements nested within another SQL statement. They can appear in SELECT, FROM, WHERE, and HAVING clauses.
Learning objectives
- Explain the difference between a correlated and a non-correlated subquery, in terms of when each one re-runs.
- Explain why NOT IN silently returns zero rows when the subquery contains a NULL, and how to avoid that trap.
- Decide, for a given query, whether a subquery or an equivalent JOIN is the better choice.
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Subqueries, EXISTS & Correlated Subqueries