Answer
A correlated subquery refers to values from an outer query level. • Its meaning depends on the current outer row. • The optimizer may rewrite it as a join or another plan. • Correlation can be clear for EXISTS checks but should still be measured for performance.
💡 SQL Example
SELECT c.name FROM customers c WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.customer_id);
Result
Customers that have at least one order
⚡ Quick Revision
A correlated subquery uses values from its outer query.