What is EXISTS?

Check whether a subquery returns any row.

EXISTS checks whether a subquery returns at least one row. It returns true or false.

SELECT name FROM customers c
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id);

This query returns customers with orders.

name is selected from customers c. WHERE keeps only the rows where EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id) holds.

What is EXISTS? — details

EXISTS is a correlated subquery. It stops at the first matching row, so it is fast.

It doesn't matter which columns the subquery returns; that's why SELECT 1 is usually written and only the existence of a row is checked.

Find customers with orders using EXISTS.

Show example solution

SELECT name FROM customers c
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id);

Add EXISTS

SELECT name FROM customers c
WHERE ____ (SELECT 1 FROM orders o WHERE o.customer_id = c.id);

Show answer

SELECT name FROM customers c
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id);

Next lesson: NOT EXISTS