"Frankenrows"

  • I'm not sure if that's the correct term for what we're seeing here, but it seems appropriate. Here's some code:

    CREATE TABLE #Details
    (
    ID INT,
    ColumnA INT,
    ColumnB INT
    )

    CREATE TABLE #SubDetails
    (
    A INT,
    B INT
    )

    INSERT INTO #SubDetails (A, B)
    VALUES (1, NULL), (2, 2)

    INSERT INTO #Details (ID)
    VALUES (1), (2), (3)

    -- This returns the dataset that is about to be updated.
    SELECT D.ID,
    ColumnA = SUB.A,
    ColumnB = SUB.B
    FROM #Details D
    JOIN #SubDetails SUB ON (1 = 1)

    UPDATE D
    SET ColumnA = SUB.A,
    ColumnB = SUB.B
    FROM #Details D
    JOIN #SubDetails SUB ON (1 = 1)

    SELECT * FROM #Details

    DROP TABLE #Details
    DROP TABLE #SubDetails

    We're doing an UPDATE on a dataset where there's more than one matching row for each row being updated. The first dataset returned above is

    Grid 1

    My understanding (mistaken, clearly) is that for each ID, SQL could chose either "1 and NULL" or "2 and 2" depending on who-knows-what.

    However, what actually happens is

    Grid 2

    where is seems to have picked the value from one column in one row and the second column from a different row.

    Does anybody know what's going on here? Thanks.

    • This topic was modified 1 month, 2 weeks ago by  julian.fletcher. Reason: Adding tags, improving formatting
  • ON (1 =1) is unusual JOIN syntax – can you explain what you want it to do?

    If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.

  • Phil Parkin wrote:

    ON (1 =1) is unusual JOIN syntax – can you explain what you want it to do?

    That's what people where I work at did before they knew how to spell CROSS JOIN.

    And, it was intentional... that's what they actually wanted to do was a CROSS JOIN but didn't know.

    --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".

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

  • You get the same results with "CROSS JOIN #SubDetails SUB".

    The code I posted is an abstraction / summary of some production code we were investigating recently. I'm just interested to know what's going on as it's not something I understand. (One of many things I don't understand!)

  • It seems to UPDATE with the first non null value it comes across.

    Due to the way SQL Server is internally structured, with a small heap under low load this will probably be the order the data was inserted. The result will actually be nondeterministic as in theory a table is an unordered set and Microsoft documentation states that the order cannot be guaranteed. (So far the 'Quirky Update' can get around this with a lot of rules but this has nothing to do with your code.) ie The production code is probably wrong and causing obscure intermittent errors. I would try and find out what the code is meant to do and implement it in a deterministic manner.

    This is also why ANSI SQL does not support an UPDATE with a JOIN and why MERGE throws an exception if there is an attempt to update the same row more than once. In tsql, given that MERGE can cause a lot of blocking, it is fine to use an UPDATE with a JOIN but you need to ensure the updated row only joins to one row in the other table as, unlike MERGE, there is no exception if multiple rows occur.

    • This reply was modified 1 month, 2 weeks ago by  Ken McKelvey.
  • Try a CROSS APPLY with a TOP(1) instead of a join.

    CREATE TABLE #Details
    (
    ID INT,
    ColumnA INT,
    ColumnB INT
    )

    CREATE TABLE #SubDetails
    (
    A INT,
    B INT
    )

    INSERT INTO #SubDetails (A, B)
    VALUES (1, NULL), (2, 2)

    INSERT INTO #Details (ID)
    VALUES (1), (2), (3)

    -- This returns the dataset that is about to be updated.
    SELECT D.ID,
    ColumnA = SUB.A,
    ColumnB = SUB.B
    FROM #Details D
    JOIN #SubDetails SUB ON (1 = 1)

    UPDATE d
    SET ColumnA = SUB.A,
    ColumnB = SUB.B
    FROM #Details AS d
    CROSS APPLY
    (
    SELECT TOP(1) *
    FROM #SubDetails AS sub
    ORDER BY sub.a DESC /* Change the ORDER BY as desired. */
    ) AS sub

    SELECT * FROM #Details

    DROP TABLE #Details
    DROP TABLE #SubDetails

    Drew

    J. Drew Allen
    Business Intelligence Analyst
    Philadelphia, PA

  • julian.fletcher wrote:

    You get the same results with "CROSS JOIN #SubDetails SUB".

    The code I posted is an abstraction / summary of some production code we were investigating recently. I'm just interested to know what's going on as it's not something I understand. (One of many things I don't understand!)

    I'm still curious... is there actually a WHERE (1=1) in the production code or not?

    --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".

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

  • We have about 160 instances of "WHERE (1 = 1)" or "ON (1 = 1)" in our code base and 950 of "CROSS JOIN".

    The code base has about 2.6m lines of code (including commenting).

    But the execution plans seem to be the same, in this case at least, and that's not really "the interesting bit".

    I was just wondering if anybody could explain how we end up with a "1" in ColumnA and a "2" in ColumnB where the source data doesn't have any row with both a "1" and a "2" in it. It's an academic question, prompted by but not replicated in our production code. I'm just interested to know what's going on under the bonnet as it were.

  • julian.fletcher wrote:

    We have about 160 instances of "WHERE (1 = 1)" or "ON (1 = 1)" in our code base and 950 of "CROSS JOIN".

    The code base has about 2.6m lines of code (including commenting).

    But the execution plans seem to be the same, in this case at least, and that's not really "the interesting bit".

    I was just wondering if anybody could explain how we end up with a "1" in ColumnA and a "2" in ColumnB where the source data doesn't have any row with both a "1" and a "2" in it. It's an academic question, prompted by but not replicated in our production code. I'm just interested to know what's going on under the bonnet as it were.

    Run the following and see.

       DROP TABLE IF EXISTS #TableA,#TableB;
    GO
    SELECT v.SomeInt
    INTO #TableA
    FROM (VALUES (1),(2),(3),(4))v(SomeInt)
    ;
    SELECT v.SomeInt
    INTO #TableB
    FROM (VALUES (5),(6),(7),(8))v(SomeInt)
    ;
    --===== Forms a "Cross Join" where all rows of TableB
    -- is returned for each row of TableA
    SELECT aSomeInt = a.SomeInt
    ,bSomeInt = b.SomeInt
    FROM #TableA a
    JOIN #TableB b
    ON 1=1
    ;

    The 1=1 and Cross Join stuff in your database is NOT necessarily bad.  "It Depends" on what else is being used for criteria in the JOIN and WHERE clauses.

    --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".

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

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