EXISTS
The EXISTS operator is used to test for the existence of any record in a sub query.
The EXISTS operator returns TRUE if the sub query returns one or more records.
Example
SELECT customers.customer_name
FROM customers
WHERE EXISTS (
SELECT order_id
FROM orders
WHERE customer_id = customers.customer_id
);
The result in example above showed that 89 customers had at least one order in the orders table.
NOT EXISTS
To check which customers that do not have any orders, we can use the NOT operator together with the EXISTS operator :
Example
SELECT customers.customer_name
FROM customers
WHERE NOT EXISTS (
SELECT order_id
FROM orders
WHERE customer_id = customers.customer_id
);
