Customer Table

Id	FirstName	LastName
1	Ozgur	Ozturk
2	Youssef	Medi
3	Henry	Tai

Order Table

Id	CustomerId	Amount
1	2	123.50
2	3	14.80

Get all customers with a least one order

SELECT * FROM Customer WHERE EXISTS (
    SELECT * FROM Order WHERE Order.CustomerId=Customer.Id
)

Result

Id	FirstName	LastName
2	Youssef	Medi
3	Henry	Tai

Get all customers with no order

SELECT * FROM Customer WHERE NOT EXISTS (
    SELECT * FROM Order WHERE Order.CustomerId = Customer.Id
)

Result

Id	FirstName	LastName
1	Ozgur	Ozturk

Purpose

EXISTS, IN and JOIN could sometime be used for the same result, however, they are not equals :

• EXISTS should be used to check if a value exist in another table
• IN should be used for static list
• JOIN should be used to retrieve data from other(s) table(s)