EXISTS usage in SQL

Syntax
EXISTS subquery

Parameters
Subquery
Is a restricted SELECT statement (the COMPUTE clause and the INTO keyword are not allowed)

Result type
Boolean

Result value
If the subquery contains rows, TRUE. EXISTS (select null) is returned and True is returned.

 

NOT EXISTS
Not exists is opposite to EXISTS. If the subquery does NOT return rows, the WHERE clause in not exists is satisfied. In this example, find the name of the publisher who does not publish commercial books.

 

The code is as follows:	Copy code
SELECT pub_name FROM publishers WHERE NOT EXISTS (SELECT * FROM titles WHERE pub_id = publishers. pub_id AND type = 'business ') Order by pub_name

In this example, NULL is specified in the subquery and the result set is returned. By using EXISTS, the value is still TRUE.

 

The code is as follows:	Copy code
SELECT CategoryName FROM Categories Where exists (select null) Order by CategoryName ASC

Instance

The code is as follows:	Copy code
SELECT c. CustomerId, CompanyName From mers c Where exists ( SELECT OrderID FROM Orders o WHERE o. CustomerID = cu. CustomerID)