sql if

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;