Update in Batches Never Finishes

问题

as a follow up on my question original question posted here

UPDATE in Batches Does Not End and Remaining Data Does Not Get Updated

If you use the logic below you'll see that update never finishes. Let me know if you have any ideas why...

Table 1

IF OBJECT_ID('tempdb..#Table2') IS NOT NULL
        BEGIN 
            DROP TABLE #Table2;
        END

        CREATE TABLE #Table2 (ID INT);

        DECLARE @Count int = 0;

        WHILE (select count(*) from #Table2) < 10000 BEGIN
            INSERT INTO #Table2 (ID) 
            VALUES (@Count)

            -- Make sure we have a unique id for the test, else we can't identify 10 records
            set @Count = @Count + 1;
        END

Table 2

IF OBJECT_ID('tempdb..#Table1') IS NOT NULL
    BEGIN 
        DROP TABLE #Table1;
    END

    CREATE TABLE #Table1 (ID INT);

    DECLARE @Count int = 0;

    WHILE (select count(*) from #Table1) < 5000 BEGIN
        INSERT INTO #Table1 (ID) 
        VALUES (@Count)

        -- Make sure we have a unique id for the test, else we can't identify 10 records
        set @Count = @Count + 1;
    END


     /****************** UPDATE ********************/

    select count (*) from #Table2 t2 where Exists (select * from #Table1 t1 where t1.ID = t2.ID)
    select count (*) from #Table2 where ID = 0
    select count (*) from #Table1 where ID = 0
       -- While exists an 'un-updated' record continue
    WHILE exists (select 1 from #Table2 t2 where Exists (select * from #Table1 t1 where t1.ID = t2.ID) ) 
    BEGIN

        -- Update any top 10 'un-updated' records
        UPDATE t2
        SET ID = 0
        FROM #Table2 t2
        WHERE ID IN (select top 10 id from #Table2 where Exists (select * from #Table1 t1 where t1.ID = t2.ID) )
    END

回答1:

Your UPDATE statement is referencing the wrong instance on #Table2. You want the following:

UPDATE t2 SET
    ID = 0
FROM #Table2 t2
WHERE ID IN (
    SELECT TOP 10 ID
    -- note this alias is t2a, and is what the `exists` needs to reference
    -- not the table being updated (`t2`)
    FROM #Table2 t2a
    WHERE EXISTS (SELECT 1 FROM #Table1 t1 WHERE t1.ID = t2a.ID)
)

Note: For testing ensure that @Count starts from 1 not 0 else you do still end up with an infinite loop.

来源：https://stackoverflow.com/questions/60822374/update-in-batches-never-finishes

标签

sql-server

loops

while-loop

sql-update

batch-updates