⟩ How To Test Subquery Results with the EXISTS Operator?
EXISTS is a special operator used to test subquery results. EXISTS can be used in two ways:
EXISTS (SELECT ...)
-- Returns TRUE if the specified subquery has one or more rows returned.
NOT EXISTS (SELECT ...)
-- Returns TRUE if the specified subquery no rows returned.
The following tutorial exercise shows you two examples of EXISTS operators. The sample database AdventureWorksLT provided by Microsoft is used.
USE AdventureWorksLT
GO
-- Number of customers with orders
SELECT COUNT(*) FROM SalesLT.Customer c
WHERE EXISTS (
SELECT * FROM SalesLT.SalesOrderHeader s
WHERE s.CustomerID = c.CustomerID
)
GO
32
-- Number of customers without orders
SELECT COUNT(*) FROM SalesLT.Customer c
WHERE NOT EXISTS (
SELECT * FROM SalesLT.SalesOrderHeader s
WHERE s.CustomerID = c.CustomerID
)
408
