Correlated subquery
Inner SELECT references the outer row — “am I above my department average?”
SQL drill • Subqueries
employees(id, name, dept_id, salary). List employees whose salary is greater than the average salary in their own department.
The subquery repeats for each outer row because it mentions e.dept_id.
Pattern
WHERE col > (SELECT AVG … WHERE inner.dept = outer.dept)
WHERE col > (SELECT AVG … WHERE inner.dept = outer.dept)
Step 0/5Line —
Outer scan + per-row inner average
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Correlation = inner query uses a column from the outer scope.