MySQL EXISTS

Last Updated: 02 July, 2023

In MySQL, the EXISTS keyword is used in conjunction with a subquery to check for the existence of rows that satisfy certain criteria. The subquery is typically correlated to the outer query, meaning it refers to the tables in the outer query. The EXISTS keyword returns a Boolean value (true or false) based on whether the subquery returns any rows or not.

Here's the basic syntax for using EXISTS in MySQL:

 SELECT column1, column2, ...., 
 FROM table_name
 WHERE EXISTS (subquery);

Here's an example that demonstrates the usage of EXISTS in MySQL:

The subquery can include conditions and references to tables in the outer query. If the subquery returns any rows, the EXISTS condition is considered true, and the outer query proceeds accordingly.

 SELECT *
 FROM customers
 WHERE EXISTS (
    SELECT 1
    FROM orders
    WHERE orders.customer_id = customers.id
 );

In this example, the query selects all customers who have at least one order. The EXISTS subquery checks if there is any row in the "orders" table that matches the customer ID from the outer query's "customers" table.

Note that the actual subquery and outer query conditions may vary based on our specific requirements. The key point is that the EXISTS keyword allows us to check for the existence of rows that meet certain criteria within a subquery.