Question

我需要在PostgreSQL中改进这个查询

select a.*
            ,(select num
                    from TABLEA c
                 where c.one = a.one
                     and c.two = a.two
                     and c.three = a.three
                     and c.four= a.four
                     and  c.five = 'A') as A
            ,(select num
                    from TABLEA c
                 where c.one = a.one
                     and c.two = a.two
                     and c.three = a.three
                     and c.four = a.four
                     and c.five = 'B') as B
            ,(select num
                    from TABLEA c
                 where c.one = a.one
                     and c.two = a.two
                     and c.three = a.three
                     and c.four = a.four
                     and c.five = 'C') as C
                from TABLEB a

我想只运行一个选择来获得A，B，C ....

Answer 1

我想你可以试试这个：

SELECT A.*, 
        C.A, C.B, C.C
FROM TABLEB A
LEFT JOIN (
    SELECT one, two, three, four
        , MAX( CASE WHEN five = 'A' THEN num ELSE NULL END) AS A
        , MAX( CASE WHEN five = 'B' THEN num ELSE NULL END) AS B
        , MAX( CASE WHEN five = 'C' THEN num ELSE NULL END) AS C
    FROM TABLEA 
    WHERE five IN ('A','B','C')
    GROUP BY one, two, three, four
) C ON  c.one = a.one
                     and c.two = a.two
                     and c.three = a.three
                     and c.four= a.four;

输出：

    one two three   four    a       b   c
1   1   2   3       4       10      20  30
2   5   6   7       8       NULL    100 NULL

Answer 2

尝试这样：

select 
(CASE WHEN five = 'A' THEN num ELSE NULL END),
(CASE WHEN five = 'B' THEN num ELSE NULL END),
(CASE WHEN five = 'C' THEN num ELSE NULL END)
from TABLEA

改进PostgreSQL中的查询

2 个答案: