Code Inspection: Unused subquery item

Reports columns, aliases, and other subquery items that are not referenced in the outer query expression.

Example (PostgreSQL):

CREATE TABLE for_subquery(id INT);
SELECT a, q FROM (SELECT 1 AS a, 10 AS b, 2 + 3 AS q, id
      FROM for_subquery) x;

We reference a and q aliases from a subquery. But the b alias and the id column are not referenced in the outer SELECT statement. Therefore, b and id are grayed out.