分组查询

select 列1,列2,聚合... from 表名 group by 列1,列2,列3...

select gender as 性别,count(*)
from students
group by gender;

select hometown as 家乡,count(*)
from students
group by hometown;

select 列1,列2,聚合... from 表名
group by 列1,列2,列3...
having 列1,...聚合...

方案一
select count(*)
from students
where gender=1;
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方案二：
select gender as 性别,count(*)
from students
group by gender
having gender=1;