Subqueries - EXISTS operator

Subqueries - NOT EXISTS operator

Let’s say we want to get the list of books that belong to a course which doesn't have any students enrolled in it.

If we break down the ask in this case, it simply means selecting records from a table with a criteria that no records related to a parameter from that table exists in the results of another query.

This is the exact opposite use case of EXISTS and adding a NOT before EXISTS solves the problem for us. Take a look at the query below.