Sql 为什么这个查询有两个选择？

我有一个疑问：

SELECT         WorkId, RegisterDate, sum(RoomType1) As RoomType1, sum(RoomType2) As RoomType2, sum(RoomType3) As RoomType3, sum(RoomType4) As RoomType4, sum(RoomType5) As RoomType5, sum(RoomType6) As RoomType6, sum(RoomType7) As RoomType7, sum(RoomType8) As RoomType8
FROM (

SELECT     dbo.[Work].WorkId, dbo.[Work].RegisterDate, 

case dbo.Floor.RoomType when 1 then 1 else 0 end as RoomType1,
        case dbo.Kat.RoomType when 2 then 1 else 0 end as RoomType2,


FROM            dbo.Belediye INNER JOIN
                         dbo.[Is] ON dbo.Municipality.MunicipalityId= dbo.[Is].MunicipalityWorkId INNER JOIN
                         dbo.Look ON dbo.[Work].LookWorkId = dbo.Look.LookId ,
WHERE        (dbo.Look.LocationIS NOT NULL)

) E
GROUP BY WorkId, 

---

这个查询按预期工作，但我不明白为什么它有两个选择，为什么它需要它们？请给我解释一下。谢谢。

由于您怀疑此查询不需要两次选择，因此可以在不使用子查询的情况下重写：

SELECT  i.IsId, 
        i.KayitTarihi, 
        SUM(case k.OdaTipi when 1 then 1 else 0 end) as RoomType1,
        SUM(case k.OdaTipi when 2 then 1 else 0 end) as RoomType2,
        SUM(case k.OdaTipi when 3 then 1 else 0 end) as RoomType3,
        SUM(case k.OdaTipi when 4 then 1 else 0 end) as RoomType4,
        SUM(case k.OdaTipi when 5 then 1 else 0 end) as RoomType5,
        SUM(case k.OdaTipi when 6 then 1 else 0 end) as RoomType6,
        SUM(case k.OdaTipi when 7 then 1 else 0 end) as RoomType7,
        SUM(case k.OdaTipi when 8 then 1 else 0 end) as RoomType8
FROM dbo.Belediye b
INNER JOIN dbo.[Is] i
    ON b.BelediyeId = i.BelediyeIsId 
INNER JOIN dbo.YerGorme yg
    ON i.YerGormeIsId = yg.YerGormeId 
INNER JOIN dbo.Kat k
    ON yg.YerGormeId = k.YerGorme_YerGormeId
WHERE yg.Lokasyon IS NOT NULL
GROUP BY i.IsId, i.KayitTarihi

---

注意：使用

内部选择称为派生表。执行

分组时非常方便，可以避免重复编写复杂表达式。（根据ANSI SQL，这甚至是不允许的。）在这种情况下似乎并不需要它。您可以在一次选择中进行分组，然后执行
sum（case…end）
操作。