SQL自连接多次

Question

我有一个存储星期条目的数据库表。

Id        Value     WeekId
1         1.0000    1
2         2.0000    1

同一星期最多可以有三个条目。

所以我想使用自我联接可以解决这个问题

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

问题：使用最大条目数时，它会很糟糕，但是不会回退包含一两个条目的行。

我是否可以使用其他类型的联接来拉回仅包含一个或两个条目的行，或者采用不同的方式来实现这一点？

Answer 1

这些条目不会返回，因为当联接的表返回NULL值时，您的WHERE子句会明确过滤掉它们。

此解决方案向每个记录添加一个连续的行号，然后每周重新开始为1。 这使您可以在PIVOT语句中使用此序列号

SQL 2000语句

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语句

;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

您将需要在where子句中添加w2.Id is null或w3.id is null的可能性

所以像

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

您也可以使用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