SQL Subquery Clause for Checking Existence of Matching Records

A. 'Exists'

The 'EXISTS' clause in SQL is used to check whether a subquery returns at least one matching record. If the subquery returns at least one row, the 'EXISTS' clause evaluates to 'TRUE'. Otherwise, it evaluates to 'FALSE'.

Example:

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

This query will return all customers who have placed at least one order. The subquery checks if there is an order record associated with the customer's ID. If there is, the 'EXISTS' clause evaluates to 'TRUE' and the customer is included in the result set.