Question

I’ve seen people hypothetically say that there are cases when a subquery can be more efficient than a join but I have never actually seen a good example of this?

What would be a case when you would want to use a subquery over a join?

Answer 1

The classic example is searching for rows in a table that do not have corresponding rows in another.

SELECT a.*
    FROM TableA a
    WHERE NOT EXISTS(SELECT NULL FROM TableB b WHERE b.parent_id = a.id)

is generally better than

SELECT a.* 
    FROM TableA a
        LEFT JOIN TableB b
            ON a.id = b.parent_id
    WHERE b.parent_id IS NULL

See also: Left outer join vs NOT EXISTS