MySQL联表查询中left-join常见问题

DROP TABLE IF EXISTS `role`;
CREATE TABLE `role` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `role_name` VARCHAR(50) DEFAULT NULL COMMENT '角色名',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='角色表';


insert into `role` VALUES(1, '管理员');
insert into `role` VALUES(2, '总经理');
insert into `role` VALUES(3, '科长');
insert into `role` VALUES(4, '组长');

DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `role_id` int(11) NOT NULL COMMENT '角色id',
 `user_name` VARCHAR(50) DEFAULT NULL COMMENT '用户名',
 `sex` int(1) DEFAULT 0 COMMENT '性别',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='用户表';

insert into `user` VALUES(1, 1, 'admin', 1);
insert into `user` VALUES(2, 2, '王经理', 1);
insert into `user` VALUES(3, 2, '李经理', 2);
insert into `user` VALUES(4, 2, '张经理', 2);
insert into `user` VALUES(5, 3, '王科长', 1);
insert into `user` VALUES(6, 3, '李科长', 1);
insert into `user` VALUES(7, 3, '吕科长', 2);
insert into `user` VALUES(8, 3, '邢科长', 1);
insert into `user` VALUES(9, 4, '范组长', 2);
insert into `user` VALUES(10, 4, '赵组长', 2);
insert into `user` VALUES(11, 4, '姬组长', 1);