MySQL联表查询基本操作之left-join常见的坑

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);

mysql> select * from role;
+----+-----------+
| id | role_name |
+----+-----------+
| 1 | 管理员  |
| 2 | 总经理  |
| 3 | 科长   |
| 4 | 组长   |
+----+-----------+
4 rows in set (0.00 sec)

mysql> select * from user;
+----+---------+-----------+------+
| id | role_id | user_name | sex |
+----+---------+-----------+------+
| 1 |    1 | admin   |  1 |
| 2 |    2 | 王经理  |  1 |
| 3 |    2 | 李经理  |  2 |
| 4 |    2 | 张经理  |  2 |
| 5 |    3 | 王科长  |  1 |
| 6 |    3 | 李科长  |  1 |
| 7 |    3 | 吕科长  |  2 |
| 8 |    3 | 邢科长  |  1 |
| 9 |    4 | 范组长  |  2 |
| 10 |    4 | 赵组长  |  2 |
| 11 |    4 | 姬组长  |  1 |
+----+---------+-----------+------+
11 rows in set (0.00 sec)

mysql> SELECT
  -> id,
  -> user_name AS '姓名',
  -> ( CASE WHEN sex = 1 THEN '男' WHEN sex = 2 THEN '女' ELSE '未知' END ) AS '性别'
  -> FROM
  -> USER;
+----+-----------+--------+
| id | 姓名   | 性别  |
+----+-----------+--------+
| 1 | admin   | 男   |
| 2 | 王经理  | 男   |
| 3 | 李经理  | 女   |
| 4 | 张经理  | 女   |
| 5 | 王科长  | 男   |
| 6 | 李科长  | 男   |
| 7 | 吕科长  | 女   |
| 8 | 邢科长  | 男   |
| 9 | 范组长  | 女   |
| 10 | 赵组长  | 女   |
| 11 | 姬组长  | 男   |
+----+-----------+--------+

mysql> SELECT
  -> r.id,
  -> r.role_name AS role,
  -> count( u.sex ) AS sex
  -> FROM
  -> role r
  -> LEFT JOIN USER u ON r.id = u.role_id
  -> AND u.sex = 2
  -> GROUP BY
  -> r.role_name
  -> ORDER BY
  -> r.id ASC;
+----+-----------+-----+
| id | role   | sex |
+----+-----------+-----+
| 1 | 管理员  |  0 |
| 2 | 总经理  |  2 |
| 3 | 科长   |  1 |
| 4 | 组长   |  2 |
+----+-----------+-----+
4 rows in set (0.00 sec)

mysql> SELECT
  -> r.id,
  -> r.role_name AS role,
  -> count( u.sex ) AS sex
  -> FROM
  -> role r
  -> LEFT JOIN USER u ON r.id = u.role_id
  -> WHERE
  -> u.sex = 2
  -> GROUP BY
  -> r.role_name
  -> ORDER BY
  -> r.id ASC;
+----+-----------+-----+
| id | role   | sex |
+----+-----------+-----+
| 2 | 总经理  |  2 |
| 3 | 科长   |  1 |
| 4 | 组长   |  2 |
+----+-----------+-----+
3 rows in set (0.00 sec)

mysql> SELECT
  -> r.id,
  -> r.role_name AS role,
  -> count( u.sex ) AS sex
  -> FROM
  -> role r
  -> LEFT JOIN USER u ON r.id = u.role_id
  -> WHERE
  -> r.role_name = '总经理'
  -> GROUP BY
  -> r.role_name
  -> ORDER BY
  -> r.id ASC;
+----+-----------+-----+
| id | role   | sex |
+----+-----------+-----+
| 2 | 总经理  |  3 |
+----+-----------+-----+
1 row in set (0.00 sec)

mysql> SELECT
  -> r.id,
  -> r.role_name AS role,
  -> count( u.sex ) AS sex
  -> FROM
  -> role r
  -> LEFT JOIN USER u ON r.id = u.role_id
  -> AND r.role_name = '总经理'
  -> GROUP BY
  -> r.role_name
  -> ORDER BY
  -> r.id ASC;
+----+-----------+-----+
| id | role   | sex |
+----+-----------+-----+
| 1 | 管理员  |  0 |
| 2 | 总经理  |  3 |
| 3 | 科长   |  0 |
| 4 | 组长   |  0 |
+----+-----------+-----+
4 rows in set (0.00 sec)