一共有3张表, user , role , relate(关联表)
user 表字段 userid,username,
role 表 字段 roleid,rolename,rolecode,
relate 表字段 userid,roleid,已经主外键关联了
我想查询 username,rolename,rolecode.应该怎么查询？
select uername,rolename,rolecode from user,role,relate where user.userid = relate.userid and relate.roleid = role.roleid;
select a.username,b.rolename,b.rolecode from user a,role b,relate c where c. userid=a.userid(+) and c.roleid=b.roleid(+)
select t1.username,t3.rolename,t3.rolecode
from user t1 inner join relate t2 on t1.userid=t2.userid
inner join role t3 on t2.roleid=t3.roleid;
参考自：
mysql join语法 http://www.data.5helpyou.com/article209.html