合并-仅在值更改时更新 [英] Merge - Only update if values have changed

问题描述

我正在SQL Server中运行合并.在我的更新中，我只想更新值已更改的行.版本行在每次更新时都会递增.下面是一个示例:

I am running a merge in SQL Server. In my update, I want to only update the row if the values have changed. There is a version row that increments on each update. Below is an example:

MERGE Employee as tgt USING 
(SELECT Employee_History.Emp_ID
, Employee_History.First_Name
, Employee_History.Last_Name
FROM Employee_History)
as src (Emp_ID,First_Name,Last_Name)
ON tgt.Emp_ID = src.Emp_ID
WHEN MATCHED THEN 
    UPDATE SET
    Emp_ID = src.Emp_ID,
    ,[VERSION] = tgt.VERSION + 1 
    ,First_Name = src.First_Name
    ,Last_Name = src.Last_Name
WHEN NOT MATCHED BY target THEN 
    INSERT (Emp_ID,0,First_Name,Last_Name)
VALUES 
    (src.Emp_ID,[VERSION],src.First_Name,src.Last_Name);

现在，如果我只想更新行，从而增加版本，则仅在名称更改的情况下.

Now, if I only wanted to update the row, and thus increment version, ONLY if the name has changed.

推荐答案

WHEN MATCHED可以具有AND.另外，无需更新EMP_ID.

WHEN MATCHED can have AND . Also, no need to update EMP_ID .

...
 WHEN MATCHED AND (trg.First_Name <> src.First_Name 
   OR trg.Last_Name <> src.Last_Name) THEN UPDATE
   SET 
   [VERSION] = tgt.VERSION + 1 
    ,First_Name = src.First_Name
    ,Last_Name = src.Last_Name
 ...

如果Last_Name或First_Name可为空，则在比较trg.Last_Name<> src.Last_Name时，例如ISNULL(trg.Last_Name,'') <> ISNULL(src.Last_Name,'')

If Last_Name or First_Name are nullable, you need to take care of NULL values while comparing trg.Last_Name <> src.Last_Name , for instance ISNULL(trg.Last_Name,'') <> ISNULL(src.Last_Name,'')

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