Question

假设我有一个这样的表：

Letter  Color  
A       Red    
A       Blue    
B       Red          
C       Red
C       Red

我想要实现的是以下输出格式：

Letter   Red   Blue
A        1     1
B        1     0
C        2     0

尝试以下：

SELECT letter, red, blue FROM (
SELECT letter, count(*) AS red from letters where color = 'red'
GROUP BY letter
UNION
SELECT letter, count(*) AS blue from letters where color = 'blue'
GROUP BY letter
) GROUP BY letter

有人可以帮忙吗？请注意，我不能添加任何tablefunc或类似于DB的表。预先感谢

Answer 1

我认为最简单的方法是条件聚合：

select letter,
       sum(case when color = 'Red' then 1 else 0 end) as red,
       sum(case when color = 'Blue' then 1 else 0 end) as blue
from letters
group by letter
order by letter;

Answer 2

您可以UNION和SELECT作为红色，蓝色值，并获得红色，蓝色MAX()的{{1}}值

GROUP BY letter

Answer 3

您可以尝试对Letter进行分组以分别获取计数，然后加入

select T1.Letter, CASE WHEN Red is NULL THEN 0 ELSE Red END AS Red , CASE WHEN Blue is NULL THEN 0 ELSE Blue END AS Blue   
from 
(select Letter, count(*) as Red from letters where Color = 'Red' group by Letter) T1 
full join 
(select Letter, count(*) as Blue from letters where Color = 'Blue' group by Letter) T2 
on T1.Letter = T2.Letter;

PSQL按同一行上的多个条件进行分组

3 个答案: