在DB2中旋轉

Question

我必須將行轉換為DB2表中的列。這就是表的結構。

ItemID    Item    Value
---------------------
1     Meeting     Now
1     Advise      Yes
1     NoAdvise    No
2     Meeting     Never
2     Advise      No
2     NoAdvise    Null
2     Combine    Yes

我希望將其轉置為（請注意，我不想轉置合並）

ItemID    Meeting  Advise   NoAdvise 
---------------------------------------
1         Now      Yes       No
2         Never    No        Null

有點掙扎的查詢，請您幫忙嗎？

Answer 1

它不是很漂亮，但是應該可以。 DB2沒有像SQL Server這樣的內置PIVOT功能。

SELECT DISTINCT
     A.ItemID
    ,(SELECT value
      FROM table B
      WHERE B.ItemID = A.ItemID
        AND B.Item   = 'Meeting'
    ) AS Meeting
    ,(SELECT value
      FROM table B
      WHERE B.ItemID = A.ItemID
        AND B.Item   = 'Advise'
    ) AS Advise
    ,(SELECT value
      FROM table B
      WHERE B.ItemID = A.ItemID
        AND B.Item   = 'NoAdvise'
    ) AS NoAdvise
FROM table A

Answer 2

bhamby當前接受的答案當然是正確的，但是值得檢查使用多個相關子查詢是否比單個查詢慢得多 （提示：很可能是）：

SELECT 
  A.ItemID,
  MAX(CASE WHEN A.Item = 'Meeting'  THEN Value END) AS Meeting,
  MAX(CASE WHEN A.Item = 'Advise'   THEN Value END) AS Advise,
  MAX(CASE WHEN A.Item = 'NoAdvise' THEN Value END) AS NoAdvise
FROM A
GROUP BY A.ItemID

我認為這也更簡單

SQLFiddle （在PostgreSQL中，但也適用於DB2 LUW）

Answer 3

正如@bhamby所說，DB2沒有PIVOT功能。
通常，我的查詢在檢索結果的方式上有所不同-您需要對它們運行分析器/優化器以確保結果，但是我相信相關子查詢可能按行執行（可能效率較低），而不是集合。 對於小型數據集，這不太可能成為問題。

WITH Item (id) as (SELECT DISTINCT itemId
                   FROM YourTable),
SELECT item.id, Meeting.meeting, Advise.advise, NoAdvise.noadvise
FROM Item
LEFT JOIN (SELECT itemId, value as meeting
           FROM YourTable
           WHERE item = 'Meeting') as Meeting
       ON Meeting.itemId = Item.id
LEFT JOIN (SELECT itemId, value as advise
           FROM YourTable
           WHERE item = 'Advise') as Advise
       ON Advise.itemId = Item.id
LEFT JOIN (SELECT itemId, value as noadvise
           FROM YourTable
           WHERE item = 'NoAdvise') as NoAdvise
       ON NoAdvise.itemId = Item.id

（...，實際上，我有點擔心您同時具有“建議”和“不建議”列，這似乎是某種布爾條件-即，您應該有一個，但沒有另一個） 。