sql

select count(*) as studentcount,classroom.id as claaid,gender as claaid from student left join classroom on classroom.classnumber = student.classid left join school on student.schoolid = school.id group by classid,gender

(select count(*) as studentcount ,gender ,classid,schoolid from student group by gender) as student left join classroom on classroom.classnumber = student.classid left join school on student.schoolid = school.id

select count(*) as studentcount ,gender ,classid from student left join classroom on classroom.classnumber = student.classid group by gender

select classid , count(case when gender='m' then 1 end) as 男 ,count(case when gender = 'f' then 1 end) as 女 ,school.name from student  left join school on student.schoolid = school.id group by student.classid


select school.name, classnumber,count(*)  ,gender from student left join classroom on classroom.classnumber = student.classid left join school on student.schoolid = school.id where gender = 'm' group by classid

union

select school.name, classnumber,count(*)  ,gender from student left join classroom on classroom.classnumber = student.classid left join school on student.schoolid = school.id where gender = 'f' group by classid

select max(id) from sudent as id1, select max(id) from school as id2; 


SELECT DATEDIFF('2008-12-29','2008-12-30') AS DiffDate