SQL Server 查询多对多关系 - 如何查询？

Question

这是对我之前问题的更新：Many to many relationship

之前的解决方案效果很好，但现在我想稍微改进一下结果。我想将所有波长值放在一行中。

所以而不是下面的结果：

DateTimeID  Wavelength  SensorID
11435   1581,665    334
11435   1515,166    334
11435   1518,286    335

我想要类似的东西：

DateTimeID  Wavelength1 Wavelength2  SensorID
11435   1581,665    1515,166     334
11435   1518,286                 335

Answer 1

您可以使用以下方法将row_number() 应用于记录：

select DateTimeID,
  [1] as Wavelength1, 
  [2] as Wavelength2,
  SensorId
from
(
  select [DateTimeID], [Wavelength], [SensorID],
    row_number() over(partition by DateTimeID, SensorId
                      order by DateTimeID) rn
  from yourtable
) src
pivot
(
  max(Wavelength)
  for rn in ([1], [2])
) piv

见SQL Fiddle with Demo。

如果您将拥有未知数量的 wavelength 值，那么您可以使用动态 SQL 来生成：

DECLARE @cols AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

select @cols = STUFF((SELECT distinct ',' + QUOTENAME('Wavelength'+cast(rn as varchar(50))) 
                    from
                    (
                      select row_number() over(partition by DateTimeID, SensorId
                                              order by DateTimeID) rn
                      from yourtable
                    ) src
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,1,'')

set @query = 'SELECT DateTimeID,' + @cols + ', SensorId  from 
             (
                select [DateTimeID], [Wavelength], [SensorID],
                  ''Wavelength''+cast(row_number() over(partition by DateTimeID, SensorId
                                        order by DateTimeID) as varchar(50)) rn
                from yourtable
            ) x
            pivot 
            (
                max(Wavelength)
                for rn in (' + @cols + ')
            ) p '

execute(@query)

见SQL Fiddle with Demo