sql server - 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.