合并-仅在值更改时更新

Question

我在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：

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。

Answer 1

WHEN MATCHED可以有AND。另外，无需更新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，NULL则例如在比较trg.Last_Name <>
src.Last_Name时，需要注意值。ISNULL(trg.Last_Name,'') <> ISNULL(src.Last_Name,'')