Caution with EXCEPT

  • Comments posted to this topic are about the item Caution with EXCEPT

  • Very nice and informative article...

    This is what we do in real life as well (Use of SELECT *). Even though we hear from everybody that to avoid SELECT * always.

    So, if we make practice of writing SELECT col1, col2, col3,.... then we would not get that problem.

    Enjoy.

  • Another issue with select * is that if you have identity keys and the tables are independently generated (as in deve and staging) and an identical data entry has separate "identities" on the different boxes select * will always return all the rows.

    Instead of the Union I usually just

    select * from A

    except

    select * from B

    select * from B

    except

    select * from A

    then you can it is easier to know where the "differences" originate.

    <><
    Livin' down on the cube farm. Left, left, then a right.

  • mohd.nizamuddin (1/17/2010)


    Very nice and informative article...

    This is what we do in real life as well (Use of SELECT *). Even though we hear from everybody that to avoid SELECT * always.

    So, if we make practice of writing SELECT col1, col2, col3,.... then we would not get that problem.

    Enjoy.

    Sorry for 2 responses. More thoughts occurred as the coffee kicked in.

    I "always" πŸ˜‰ start to worry when someone used the phrase "always", there are always reasons why always is never appropriate. (Same goes with "never"). Sometimes it is just the thing to do.

    When needing to "select col1, col2, etc." I use the Management Studio tactic of "Script table as->Select into->new query window" and then just hack out any columns I do not want. It "Always" works for me.

    <><
    Livin' down on the cube farm. Left, left, then a right.

  • Here's a better way:

    SELECT 'IN A NOT B' location, a, b, c, d, ...

    FROM tablea

    EXCEPT

    SELECT 'IN A NOT B' location, a, b, c, d, ...

    FROM tableb

    UNION ALL

    SELECT 'IN B NOT A' location, a, b, c, d, ...

    FROM tableb

    EXCEPT

    SELECT 'IN B NOT A' location, a, b, c, d, ...

    FROM tablea

    No parentheses this way.

  • Stephen Hirsch (1/18/2010)


    Here's a better way:

    SELECT 'IN A NOT B' location, a, b, c, d, ...

    FROM tablea

    EXCEPT

    SELECT 'IN A NOT B' location, a, b, c, d, ...

    FROM tableb

    UNION ALL

    SELECT 'IN B NOT A' location, a, b, c, d, ...

    FROM tableb

    EXCEPT

    SELECT 'IN B NOT A' location, a, b, c, d, ...

    FROM tablea

    No parentheses this way.

    Wait a minute! If ever there was a case for parentheses, it's right here - what was the intent of the programmer? Parentheses make the intent self-documenting:

    SELECT 'IN A NOT B' location, a, b, c, d, ...

    FROM tablea

    EXCEPT

    (SELECT 'IN A NOT B' location, a, b, c, d, ...

    FROM tableb

    UNION ALL

    SELECT 'IN B NOT A' location, a, b, c, d, ...

    FROM tableb

    EXCEPT

    SELECT 'IN B NOT A' location, a, b, c, d, ...

    FROM tablea)

    Cheers

    ChrisM

    β€œWrite the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw

    For fast, accurate and documented assistance in answering your questions, please read this article.
    Understanding and using APPLY, (I) and (II) Paul White
    Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

  • The programmer "intended" it to work the way the programmer thought they were programming it to work. πŸ˜€ [this said in the universal sense]

    <><
    Livin' down on the cube farm. Left, left, then a right.

  • Tobar (1/18/2010)


    The programmer "intended" it to work the way the programmer thought they were programming it to work. πŸ˜€ [this said in the universal sense]

    LOL you used to work there too!

    β€œWrite the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw

    For fast, accurate and documented assistance in answering your questions, please read this article.
    Understanding and using APPLY, (I) and (II) Paul White
    Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

  • We have seen poor performance with EXCEPT on large data sets.

  • Thanks for the article, I've never used EXCEPT because I didn't realize it existed.

    On the point of poor performance with large datasets, how can we expect anything else? Probably too many columns to put in a covering index. SQL has to compare every column to every column, it's going to either table scan or bookmark lookup every row. Give your SQL Server a break! I suppose when I think about it, you could include a WHERE statement to break your large datasets in to smaller ones if appropriate to what you are looking for.

  • That's interesting

    I benchmarked EXCEPT vs WHERE NOT IN (SELECT .....), and for my dataset, server, DB version etc. the performance results were equivalent, or biased towards EXCEPT.

    Can you give more details about your scenario?

  • I find that an easier way to get the list of columns is to drag them from the object explorer in SSMS. If you expand the table object's columns list, you can then drag the "Columns" parent entry into a query window, to get the comma seperated list of columns.

    This works for some other items too.

  • Bravo! What a great catch. You have my 5 star vote.

    -Mike

  • jdurandt (1/18/2010)


    That's interesting

    I benchmarked EXCEPT vs WHERE NOT IN (SELECT .....), and for my dataset, server, DB version etc. the performance results were equivalent, or biased towards EXCEPT.

    Can you give more details about your scenario?

    Can you post your benchmark code, please?

    --Jeff Moden


    RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
    First step towards the paradigm shift of writing Set Based code:
    ________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.
    "Change is inevitable... change for the better is not".
    "Dear Lord... I'm a DBA so please give me patience because, if you give me strength, I'm going to need bail money too!"

    Helpful Links:
    How to post code problems
    How to Post Performance Problems
    Create a Tally Function (fnTally)

  • Nicely done, Stephen. Well written and straight to the point.

    --Jeff Moden


    RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
    First step towards the paradigm shift of writing Set Based code:
    ________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.
    "Change is inevitable... change for the better is not".
    "Dear Lord... I'm a DBA so please give me patience because, if you give me strength, I'm going to need bail money too!"

    Helpful Links:
    How to post code problems
    How to Post Performance Problems
    Create a Tally Function (fnTally)

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