可以重用子查询吗？

Question

我在尝试执行查询时遇到了一些问题。我有两个表，一个表包含元素信息，另一个表包含与第一个表的元素相关的记录。想法是在同一行中获取元素信息以及多个记录信息。

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']

这是我想要实现的：

 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']

以下查询可获得正确的结果，但对我而言似乎效率极低，必须为每个列迭代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

您可以将聚合取出到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，这实际上是什么好处（甚至是受支持的），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