SQL自连接多次

Question

我有一个存储周条目的数据库表。

Id        Value     WeekId
1         1.0000    1
2         2.0000    1

同一周最多可以有三个条目。

所以我认为使用自连接可以解决这个

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：它可以在最大数量的条目下正常工作，但是它不会拉回具有一两个条目的行。

是否有不同类型的连接可以用来拉回只有一个或两个条目的行，或者是否有不同的方法来解决这个问题？

Answer 1

这些条目不会返回，因为当连接表返回 NULL 值时，您的 WHERE 子句会显式过滤掉它们。

此解决方案向每条记录添加一个连续的行号，每周重新开始为 1。 中使用此序列号

这允许您在 PIVOT 语句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

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

所以类似

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

您还可以使用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