Databases · Junior · Question 74 of 75

What does `EXISTS` check?

SELECT name FROM users u
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.user_id = u.id);

1Whether the table orders exists
✓Correct answer: Whether the subquery returns at least one row; here, users with at least one order
3Whether the column user_id is not NULL

Explanation

The database can stop at the first matching row. NOT EXISTS gives the opposite: users without orders.

Further reading

PostgreSQL: Subquery expressions (EXISTS, IN) ↗

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