user(UserID,UserName,UserNote)
role(RoleID,RoleName,RoleNote)
permission(PermissionID,PermissionName)
user_role(UserRoleID,UserID,RoleID)
role_permission(RolePermissionID,RoleID,PermissionID)
1.多表查询，主键、外键
string sql="select UserName,RoleName ,user.UserID from user,role,user_role
where user.UserID=user_role.UserID and role.RoleID=user_role.RoleID";
//查询角色名称，权限表，角色_权限表，用户_角色表
string sql="select PermissionName from permission,role_permission ,user_role
where user_role.UserID='"+TID+"' and role_permission.RoleID=user_role.RoleID and permission.PermissionID=role_permission.PermissionID";
注：三表查询，条件是外键
