select sex, if (sex='男',0,1) from student;
select sum(if(sex='男'm1,0)),sum(if(sex='‘ from student;
select id,ifnull(scroe,0) from scores;
select sex, if (sex='男',0,1) from student;
select sum(if(sex='男'm1,0)),sum(if(sex='‘ from student;
select id,ifnull(scroe,0) from scores;