select
vtiger_users.id,
vtiger_users.user_name,
vtiger_role.rolename
FROM
vtiger_users
inner join vtiger_role
inner join vtiger_user2role
on vtiger_users.id = vtiger_user2role.userid
and vtiger_user2role.roleid = vtiger_role.roleid
select 字段1,字段2,字段3 from 表1 inner join 表2 inner join 表3 on 表1.字段 = 表2.字段 and 表2.字段 = 表3.字段