What is a correlated subquery?

A correlated subquery references a column from the outer query, so it is logically re-evaluated for every outer row. Typical use: compare each row to a value for its own group, like each salary against its department average (shown here). It can be slow on large tables if the database cannot optimize it away. Often rewritable as a JOIN to a pre-aggregated subquery, or as a window function.

Why does NOT IN return no rows?

If the subquery returns even one NULL, NOT IN returns no rows at all. x NOT IN (1, NULL) evaluates to unknown, never true. Use NOT EXISTS instead (shown here); it is NULL-safe. Or add WHERE col IS NOT NULL inside the subquery.

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SQL Subqueries Practice Questions

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Nest one query inside another when you need its answer first.

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Scalar subquery: Returns one value, usable in SELECT or a comparison.
Correlated subquery: Refers to the outer row and runs once per row.

16 questionsPage 2 of 2

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HR wants to identify individual contributors -- employees nobody reports to. Using a correlated NOT EXISTS subquery...
SubqueriesCorrelated SubqueryNOT EXISTSAnti-Join
Intermediate
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SQL Subqueries practice: common questions

Short answers to what people get stuck on most. Tap a question to expand it.

Should I use a subquery or a JOIN?

Both can often give the same result. Choose by readability and by which columns you need.

JOIN: when you need columns from both tables in the result.
Subquery: when you only filter by, or compute one value from, the other table.

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SQL Practice Hub

SQL Subqueries Practice Questions

Leaderboard

Nest one query inside another when you need its answer first.

16 questionsFreeRuns in your browser

Scalar subquery: Returns one value, usable in SELECT or a comparison.
Correlated subquery: Refers to the outer row and runs once per row.

16 questionsPage 2 of 2

Employees With No Direct Reports (Anti-Join)
HR wants to identify individual contributors -- employees nobody reports to. Using a correlated NOT EXISTS subquery...
SubqueriesCorrelated SubqueryNOT EXISTSAnti-Join
Intermediate
6m25 pts
1100%

FAQ

SQL Subqueries practice: common questions

Short answers to what people get stuck on most. Tap a question to expand it.

Should I use a subquery or a JOIN?

Both can often give the same result. Choose by readability and by which columns you need.

JOIN: when you need columns from both tables in the result.
Subquery: when you only filter by, or compute one value from, the other table.

Track your streak & earn XP

Full-Length Interview Practice Tests100% Free

10 role-based tests for Data Analysts, SQL Developers & Data Engineers with instant scorecards.

Explore 10 Mock Tests

SQL Subqueries Practice Questions

Employees With No Direct Reports (Anti-Join)

SQL Subqueries practice: common questions

Full-Length Interview Practice Tests100% Free

SQL Subqueries Practice Questions

Employees With No Direct Reports (Anti-Join)

SQL Subqueries practice: common questions

Full-Length Interview Practice Tests100% Free

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