我有一张桌子
students[std_id, name, class, gender,etc]
select class,gender,count(*) as total_students
from students
group by class,gender
其输出如下
1st | male | 23
1st | female | 11
2nd | male | 17
2nd | female | 0
//最后一行未显示,因为二班有0名女生
如何使其如上显示,total_sudents=0而不是跳过记录。
您可以通过为每个性别编写一个查询,然后将它们联合起来来实现:
select class, 'male' as gender,
count(case when gender = 'male' then 1 end) as total_students
from students
group by class
union all
select class, 'female' as gender,
count(case when gender = 'female' then 1 end) as total_students
from students
group by class
或者,您可以执行以下操作:
select class,
count(case when gender = 'male' then 1 end) as total_male_students,
count(case when gender = 'female' then 1 end) as total_female_students
from students
group by class
使用此解决方案:
SELECT a.class,
a.gender,
COUNT(b.class) AS total_students
FROM (
SELECT a.class,
b.gender
FROM students a
CROSS JOIN (
SELECT 'male' AS gender UNION ALL
SELECT 'female'
) b
GROUP BY a.class,
b.gender
) a
LEFT JOIN students b ON a.class = b.class AND
a.gender = b.gender
GROUP BY a.class,
a.gender