我在尝试执行查询时遇到了一些问题。我有两个表,一个包含元素信息,另一个表包含与第一个表的元素相关的记录。我们的想法是在同一行中获取元素信息和几条记录信息。
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']
以下查询获得了正确的结果,但在我看来效率非常低,不得不为每列迭代table2。无论如何都可以做一个子查询并重用它?
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
答案 0 :(得分:26)
您可以将聚合带入CTE(公用表表达式):
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,这实际上是否有益(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
答案 1 :(得分:7)
一些后来的数据库产品(Oracle,SQL Server 2005,SQL Server 2008等)提供了创建公用表表达式的功能(简称CTE)。有了这个,你可以重用一个像这样的子查询:
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
当然这与以下内容没有多大区别:
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