批量更新永远不会完成

Question

作为对我在此处发布的原始问题的跟进

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

如果您使用下面的逻辑，您会发现更新永远不会完成。如果您有任何想法，请告诉我为什么...

表 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

表 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

Answer 1

您的UPDATE 语句引用了#Table2 上的错误实例。您需要以下内容：

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)
)

注意：对于测试，请确保 @Count 从 1 而不是 0 开始，否则您仍然会以无限循环结束。