select * from 表
where 条件
group by 条件
order by 排序
limit
分组 Group by
select * from 表
group by 条件
结果为每个分组的第一条记录，该条记录作为该组的标志
select * from `subject`
GROUP BY gradeid
select count(1),gradeid from `subject`
GROUP BY gradeid
count(1) ：在表后面新增加一列，全部赋值为1，再查询每组为1的个数
insert into subject (subjectname,classhour,gradeid) values(NULL,130,1);
select count(subjectname),gradeid from `subject`
GROUP BY gradeid
count(subjectname) 查询subjectname不为空的格式，如果某条记录的subjename为空，查询个数时不计入
select count(distinct subjectname),gradeid from `subject`
GROUP BY gradeid
distinct 去除分组中subjectname重复的记录
