EXISTS operator executes the outer SQL query if the subquery is not NULL (empty result-set). For example,
SELECT customer_id, first_name FROM Customers WHERE EXISTS ( SELECT order_id FROM Orders WHERE Orders.customer_id = Customers.customer_id );
Here is how the SQL command works:
This process is repeated for each row of the outer query.
Example 2: SQL EXISTS Operator
The below SQL query selects orders from the order table for customers who are older than 23 years.
SELECT * FROM Orders WHERE EXISTS ( SELECT customer_id FROM Customers WHERE Orders.customer_id = Customers.customer_id AND Customers.age > 23 );