SQL自我加入多次

Question

我有一個存儲星期條目的單個數據庫表。

Id  Value  WeekId 
1   1.0000 1 
2   2.0000 1 

同一周最多可以有三個條目。

所以我想用一個自聯接將解決這個

SELECT w1.Value, w2.Value, w3.Value 
FROM [List].[dbo].[testWeekEntries] as w1 
LEFT OUTER JOIN [List].[dbo].[testWeekEntries] as w2 ON w1.WeekId = w2.weekId 
LEFT OUTER JOIN [List].[dbo].[testWeekEntries] as w3 ON w2.WeekId = w3.WeekId 
WHERE w1.Id < w2.Id AND w2.Id < w3.Id 

的問題：但它不具有一個或兩個條目拉回來一排它worls罰款條目的最大數量。

有沒有不同類型的連接，我可以使用一個或兩個條目或不同的方式來回拉一行？

Answer 1

這些條目不會返回，因爲當連接的表返回NULL值時，您的WHERE子句會將其明確地過濾掉。

此解決方案爲每條記錄添加一個順序排列數，每週重新開始爲1。這使您可以在PIVOT語句中使用該序列號

SQL 2000語句

SELECT * 
FROM (
      SELECT (SELECT COUNT(*) 
        FROM testWeekEntries 
        WHERE Id <= we.Id 
          AND WeekId = we.WeekId) as rn 
        , Value 
        , WeekId 
      FROM testWeekEntries we 
     ) q 
PIVOT (MAX(Value) FOR rn IN ([1],[2],[3])) AS PVT 

SQL 2008語句

;WITH q AS (
SELECT rn = ROW_NUMBER() OVER (PARTITION BY WeekId ORDER BY Id) 
     , Id 
     , Value 
     , WeekId 
FROM [testWeekEntries] as w1 
) 
SELECT Value 
     , (SELECT Value FROM q q1 WHERE q1.rn = q.rn + 1 AND q1.WeekId = q.WeekId) 
     , (SELECT Value FROM q q2 WHERE q2.rn = q.rn + 2 AND q2.WeekId = q.WeekId) 
FROM q 
WHERE q.rn = 1 

Answer 2

您需要將您的WHERE子句中添加w2.Id is null或w3.id is null

因此，像

WHERE 
    (w2.Id is null and w3.id is null) or 
    (w3.id is null and w1.id < w2.id) or 
    (w1.id < w2.id and w2.id < w3.id) 

Answer 3

您還可以使用PIVOT

;WITH CTE AS 
(
SELECT Value, 
     WeekId, 
     ROW_NUMBER() OVER (PARTITION BY WeekId ORDER BY Id) AS RN 
FROM [List].[dbo].[testWeekEntries]  
) 
SELECT * 
FROM CTE 
PIVOT (MAX(Value) FOR RN IN ([1],[2],[3])) AS PVT