When to use EXCEPT as opposed to NOT EXISTS in Transact SQL?

Question

I just recently learned of the existence of the new "EXCEPT" clause in SQL Server (a bit late, I know...) through reading code written by a co-worker. It truly amazed me!

But then I have some questions regarding its usage: when is it recommended to be employed? Is there a difference, performance-wise, between using it versus a correlated query employing "AND NOT EXISTS..."?

After reading EXCEPT's article in the BOL I thought it was just a shorthand for the second option, but was surprised when I rewrote a couple queries using it (so they had the "AND NOT EXISTS" syntax much more familiar to me) and then checked the execution plans - surprise! The EXCEPT version had a shorter execution plan, and executed faster, also. Is this always so?

So I'd like to know: what are the guidelines for using this powerful tool?

Answer 1

EXCEPT treats NULL values as matching.

This query:

WITH    q (value) AS         (         SELECT  NULL         UNION ALL         SELECT  1         ),         p (value) AS         (         SELECT  NULL         UNION ALL         SELECT  2         ) SELECT  * FROM    q WHERE   value NOT IN         (         SELECT  value         FROM    p         ) 

will return an empty rowset.

This query:

WITH    q (value) AS         (         SELECT  NULL         UNION ALL         SELECT  1         ),         p (value) AS         (         SELECT  NULL         UNION ALL         SELECT  2         ) SELECT  * FROM    q WHERE   NOT EXISTS         (         SELECT  NULL         FROM    p         WHERE   p.value = q.value         ) 

will return

NULL 1 

, and this one:

WITH    q (value) AS         (         SELECT  NULL         UNION ALL         SELECT  1         ),         p (value) AS         (         SELECT  NULL         UNION ALL         SELECT  2         ) SELECT  * FROM    q EXCEPT SELECT  * FROM    p 

will return:

1 

Recursive reference is also allowed in EXCEPT clause in a recursive CTE, though it behaves in a strange way: it returns everything except the last row of a previous set, not everything except the whole previous set:

WITH    q (value) AS         (         SELECT  1         UNION ALL         SELECT  2         UNION ALL         SELECT  3         ),         rec (value) AS         (         SELECT  value         FROM    q         UNION ALL         SELECT  *         FROM    (                 SELECT  value                 FROM    q                 EXCEPT                 SELECT  value                 FROM    rec                 ) q2         ) SELECT  TOP 10 * FROM    rec  --- 1 2 3 -- original set 1 2 -- everything except the last row of the previous set, that is 3 1 3 -- everything except the last row of the previous set, that is 2 1 2 -- everything except the last row of the previous set, that is 3, etc. 1 

SQL Server developers must just have forgotten to forbid it.

Answer 2

I have done a lot of analysis of except, not exists, not in and left outer join. Generally the left outer join is the fastest for finding missing rows, especially joining on a primary key. Not In can be very fast if you know it will be a small list returned in the select.

I use EXCEPT a lot to compare what is being returned when rewriting code. Run the old code saving results. Run new code saving results and then use except to capture all differences. It is a very quick and easy way to find differences, especially when needing to get all differences including null. Very good for on the fly easy coding.

But, every situation is different. I say to every developer I have ever mentored. Try it. Do timings all different ways. Try it, time it, do it.