如果指定SELECT DISTINCT，則ORDER BY項必須出現在選擇列表中

Question

declare @userid int 
set @userid=9846 
SELECT TOP 10 
      tagid, 
      [Description] 
FROM tagsuggestion 
WHERE tagid IN (SELECT **DISTINCT** TOP 10 tagid 
      FROM (SELECT ulpt.tagid, 
          createddate 
        FROM userlocationposttag ulpt 
          WHERE ulpt.UserID=@userid         

        UNION ALL 
        SELECT ult.tagid, 
          createddate 
        FROM userlocationtag ult 
          WHERE ult.UserID=@userid 
        UNION ALL 
        SELECT upt.tagid, 
          createddate 
        FROM userprofiletag upt 
          WHERE upt.UserID=@userid 
) T 

      ORDER BY createddate DESC) 

上述查詢是錯誤的如果指定了SELECT DISTINCT，ORDER BY項必須出現在選擇列表中。

。但如果我刪除DISTINCT這個查詢運行

我要保證清晰度，我也想通過createddate DESC保持排序順序

我相關聯的支柱之一是

Last created 10 records from all of the 3 tables

Answer 1

好了，所以我們只關心此基礎上每個標籤的最新創建日期，所以我們就可以構建內部查詢爲：

SELECT tagid,MAX(createddate) as createddate FROM (
SELECT ulpt.tagid, 
     createddate 
FROM userlocationposttag ulpt 
WHERE ulpt.UserID=@userid         
UNION ALL 
SELECT ult.tagid, 
     createddate 
FROM userlocationtag ult 
WHERE ult.UserID=@userid 
UNION ALL 
SELECT upt.tagid, 
     createddate 
FROM userprofiletag upt 
WHERE upt.UserID=@userid 
) t 
GROUP BY tagid 

，然後我們可以換，在另一個子選擇，並應用前十名：

SELECT TOP 10 tagid FROM (
SELECT tagid,MAX(createddate) as createddate FROM (
SELECT ulpt.tagid, 
     createddate 
FROM userlocationposttag ulpt 
WHERE ulpt.UserID=@userid         
UNION ALL 
SELECT ult.tagid, 
     createddate 
FROM userlocationtag ult 
WHERE ult.UserID=@userid 
UNION ALL 
SELECT upt.tagid, 
     createddate 
FROM userprofiletag upt 
WHERE upt.UserID=@userid 
) t 
GROUP BY tagid 
) t ORDER BY createddate desc 

而且我們不再需要不同的（因爲GROUP BY和MAX已經確保了每個標籤只出現一次）

Answer 2

你基本上是通過告訴查詢按照不存在的字段對輸出進行排序。

您必須將createddate放置在select語句中，或者可以按tagid，[Description]和createddate進行分組，並按createddate排序。

現在，如果您覺得在包含distinct created日期時可能會獲得重複的標記和說明，那麼請圍繞該日期換一個查詢，並在這兩個字段上做前10個不同的查詢。