一个有关left join 的sql语句查询

Question

select * from student , classroom , testplace , school 
where student.roomid = classroom.id 
and classroom.testplaceid = testplace.id
and testplace.schoolid = school.id

Answer 1

select a.*,b.*,c.*,d.* from student a, classroom b, testplace c, school d
where student.roomid = classroom.id 
and classroom.testplaceid = testplace.id
and testplace.schoolid = school.id

Answer 2

slect 你要的字段
from  student join classroom on student.roomid = classroom.id 
              join testplace on classroom.testplaceid = testplace.id
              join school on testplace.schoolid = school.id

Answer 3

因为你用的是lefe join 的缘故！
要用内连接吧！
如楼上的！