SQL: … WHERE X IN (SELECT Y FROM …)

Question

Is the following the most efficient in SQL to achieve its result:

SELECT * 
  FROM Customers 
 WHERE Customer_ID NOT IN (SELECT Cust_ID FROM SUBSCRIBERS)
         


        

Answer 1

SELECT Customers.* 
  FROM Customers 
 WHERE NOT EXISTS (
       SELECT *
         FROM SUBSCRIBERS AS s
         JOIN s.Cust_ID = Customers.Customer_ID) 

When using “NOT IN”, the query performs nested full table scans, whereas for “NOT EXISTS”, the query can use an index within the sub-query.