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

If you want to know which is more effective, you should try looking at the estimated query plans, or the actual query plans after execution. It'll tell you the costs of the queries (I find CPU and IO cost to be interesting). I wouldn't be surprised much if there's little to no difference, but you never know. I've seen certain queries use multiple cores on our database server, while a rewritten version of that same query would only use one core (needless to say, the query that used all 4 cores was a good 3 times faster). Never really quite put my finger on why that is, but if you're working with large result sets, such differences can occur without your knowing about it.