可以重用子查询吗？

Question

我在尝试执行查询时遇到一些问题。我有两个表，一个包含元素信息，另一个包含与第一个表的元素相关的记录。这个想法是在同一行中获取元素信息加上几条记录信息。

结构可以这样解释：

 table [ id, name ]
 [1, '1'], [2, '2']

 table2 [ id, type, value ]
 [1, 1, '2009-12-02']
 [1, 2, '2010-01-03']
 [1, 4, '2010-01-03']
 [2, 1, '2010-01-02']
 [2, 2, '2010-01-02']
 [2, 2, '2010-01-03']
 [2, 3, '2010-01-07']
 [2, 4, '2010-01-07']

这就是我想要实现的目标：

 result [id, name, Column1, Column2, Column3, Column4]

 [1, '1', '2009-12-02', '2010-01-03', , '2010-01-03']
 [2, '2', '2010-01-02', '2010-01-02', '2010-01-07', '2010-01-07']

以下查询得到正确的结果，但在我看来效率极低，必须为每一列迭代 table2 。无论如何可以进行子查询并重用它吗？

SELECT
      a.id,
      a.name,
      (select min(value) from table2 t where t.id = subquery.id and t.type = 1 group by t.type) as Column1,
      (select min(value) from table2 t where t.id = subquery.id and t.type = 2 group by t.type) as Column2,
      (select min(value) from table2 t where t.id = subquery.id and t.type = 3 group by t.type) as Column3,
      (select min(value) from table2 t where t.id = subquery.id and t.type = 4 group by t.type) as Column4
FROM
      (SELECT distinct id
       FROM table2 t
       WHERE (t.type in (1, 2, 3, 4))
             AND t.value between '2010-01-01' and '2010-01-07') as subquery
       LEFT JOIN table a ON a.id = subquery.id

Answer 1

您可以将聚合放入 CTE（公共表表达式）中：

with minima as (select t.id, t.type, min(value) min_value
                from table2 t
                where t.type in (1,2,3,4)
                group by t.id, t.type)
select a.id, a.name,
       (select min_value from minima where minima.id = subquery.id and minima.type = 1) as column1,
       (select min_value from minima where minima.id = subquery.id and minima.type = 2) as column2,
       (select min_value from minima where minima.id = subquery.id and minima.type = 3) as column3,
       (select min_value from minima where minima.id = subquery.id and minima.type = 4) as column4
from (select distinct id from table2 t where t.type in (1,2,3,4) and t.value between '2010-01-01' and '2010-01-07') as subquery
     left join a on a.id = subquery.id

当然，这是否实际上有任何好处（甚至支持）取决于您的环境和数据集。

另一种方法：

select xx.id, a.name, xx.column1, xx.column2, xx.column3, xx.column4
from (
      select id,
             max(case type when 1 then min_value end) as column1,
             max(case type when 2 then min_value end) as column2,
             max(case type when 3 then min_value end) as column3,
             max(case type when 4 then min_value end) as column4
      from (select t.id, t.type, min(value) min_value
            from table2 t
            where t.type in (1,2,3,4)
            group by t.id, t.type) minima
      group by id
) xx left join a on a.id = xx.id
order by 1

Answer 2

后来的一些数据库产品（Oracle、SQL Server 2005、SQL Server 2008等）提供了创建公共表表达式（简称CTE）的能力。这样，您就可以像这样重用子查询：

With Subquery As
    (
    Select Id
        , Min( Case When T.TypeId = 1 Then Value End ) As MinType1
        , Min( Case When T.TypeId = 2 Then Value End ) As MinType2
        , Min( Case When T.TypeId = 3 Then Value End ) As MinType3
        , Min( Case When T.TypeId = 4 Then Value End ) As MinType4
    From Table2 As T
    Where T.Type In(1,2,3,4)
        And T.Value Between '2010-01-01' And '2010-01-07'
    Group By Id
    )
Select A.Id, A.Name, S.MinType1, S.MinType2, S.MinType3, S.MinType4
From Subquery As S
    Left Join Table As A
        On A.Id = S.Id

当然，这与以下内容没有太大不同：

Select  A.Id, A.Name, S.MinType1, S.MinType2, S.MinType3, S.MinType4
From    (
        Select Id
            , Min( Case When T.TypeId = 1 Then Value End ) As MinType1
            , Min( Case When T.TypeId = 2 Then Value End ) As MinType2
            , Min( Case When T.TypeId = 3 Then Value End ) As MinType3
            , Min( Case When T.TypeId = 4 Then Value End ) As MinType4
        From Table2 As T
        Where T.Type In(1,2,3,4)
            And T.Value Between '2010-01-01' And '2010-01-07'
        Group By Id
        ) As S
    Left Join Table As A
        On A.Id = S.Id