格式：IF(Condition,A,B)
意义：当Condition为TRUE时，返回A；当Condition为FALSE时，返回B。
例子
select A.student_id,sw,ty from
(select student_id,num as sw from score left join course on score.course_id = course.cid where course.cname =
'生物'
) as A
left join
(select student_id,num as ty from score left join course on score.course_id = course.cid where course.cname =
'体育'
) as B
on A.student_id = B.student_id where sw >
if(isnull(ty),0,ty);
如果isnull(true)为true时，返回 sw>0 否则 返回 sw> ty;
