表'dbo.UserProperties'不明確。爲什麼這個錯誤即將到來？

Question

爲什麼這個更新語句沒有被編譯？根據msdn，這是完全正確的。

UPDATE dbo.UserProperties 
    SET  Value = MergeFrom.Value 
FROM dbo.UserProperties MergeFrom , 
     dbo.UserProperties MergeTo 
WHERE MergeFrom.Field = MergeTo.Field 
     AND MergeTo.[Key] = CAST(@MergeToUserID AS NVARCHAR(50)) 
     AND MergeFrom.[Key] = CAST(@MergeFromUserID AS NVARCHAR(50)) 

Answer 1

我認爲你需要在UPDATE中引用別名來進行自我JOIN。至少我只是嘗試了一個自我JOIN的UPDATE，下面這種語法適用於我。

UPDATE MergeTo 
    SET MergeTo.Value = MergeFrom.Value 
FROM dbo.UserProperties AS MergeFrom INNER JOIN 
      dbo.UserProperties AS MergeTo ON MergeFrom.Field = MergeTo.Field 
WHERE MergeTo.[Key] = CAST(@MergeToUserID AS NVARCHAR(50)) 
         AND MergeFrom.[Key] = CAST(@MergeFromUserID AS NVARCHAR(50))