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SQLIntermediate#subqueries
SQLIntermediate#subqueries
What is a subquery? Correlated vs non-correlated?
A query nested inside another. A non-correlated subquery runs once independently. A correlated subquery references the outer row and conceptually runs per outer row, which can be slow.
Example
-- non-correlated SELECT * FROM employees WHERE salary > (SELECT AVG(salary) FROM employees); -- correlated SELECT e.name FROM employees e WHERE salary > (SELECT AVG(salary) FROM employees WHERE dept_id = e.dept_id);