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Correlated Subquery

A correlated subquery references columns of the enclosing query, so its result depends on the current outer row and it conceptually re-executes once per outer row. This per-row dependency is what distinguishes it from an independent subquery that can be evaluated once.

Questions this Concept answers

Why does a correlated subquery scale badly as the outer table grows?

jeremy Video

Correlated Subqueries in SQL That Re-run the Inner Query for Every Outer Row

A correlated subquery is a nested query whose body references a column of the enclosing (outer) query, so it cannot be evaluated independently: logically it must be re-evaluated once for every candid…