MySQL的写法：
第一种写法：
select u.name,group_concat(r.name separator ',') rolesname
from user u left join user_role ur on u.id = ur.user_id
left join role r on ur.role_id = r.id
GROUP BY u.name
第二种写法:
SELECT u.name,group_concat(r.name separator ',') rolesname
FROM user u , user_role ur, role r
WHERE u.id = ur.user_id
and r.id = ur.role_id
group by u.name;
Oracle的写法：
SELECT u.name,to_char(wm_concat(r.name)) rolesname
FROM user u , user_role ur, role r
WHERE u.id = ur.user_id
and r.id = ur.role_id
group by u.name;