代码之家 › 专栏 › 技术社区 › Gothmog

是否可以重用子查询?

query-optimization subquery sql

Gothmog · 技术社区 · 16 年前

我在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。其思想是在同一行中获取元素信息和多个记录信息。

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']

下面的查询得到了正确的结果,但在我看来效率极低,必须为每列迭代表2。无论如何都可以执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

2 回复 | 直到 10 年前

Chris 13 年前

可以将聚合提取到CTE(公用表表达式)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,这是否真的有任何好处(或者甚至不受支持)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

Thomas 16 年前

一些较新的数据库产品(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,您就可以重用这样的子查询:

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:

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