MySQL聯(lián)表查詢中l(wèi)eft-join常見問題

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, '管理員');
insert into `role` VALUES(2, '總經(jīng)理');
insert into `role` VALUES(3, '科長');
insert into `role` VALUES(4, '組長');

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 '用戶名',
 `sex` int(1) DEFAULT 0 COMMENT '性別',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='用戶表';

insert into `user` VALUES(1, 1, 'admin', 1);
insert into `user` VALUES(2, 2, '王經(jīng)理', 1);
insert into `user` VALUES(3, 2, '李經(jīng)理', 2);
insert into `user` VALUES(4, 2, '張經(jīng)理', 2);
insert into `user` VALUES(5, 3, '王科長', 1);
insert into `user` VALUES(6, 3, '李科長', 1);
insert into `user` VALUES(7, 3, '呂科長', 2);
insert into `user` VALUES(8, 3, '邢科長', 1);
insert into `user` VALUES(9, 4, '范組長', 2);
insert into `user` VALUES(10, 4, '趙組長', 2);
insert into `user` VALUES(11, 4, '姬組長', 1);

mysql> select * from role;
+----+-----------+
| id | role_name |
+----+-----------+
| 1 | 管理員  |
| 2 | 總經(jīng)理  |
| 3 | 科長   |
| 4 | 組長   |
+----+-----------+
4 rows in set (0.00 sec)

mysql> select * from user;
+----+---------+-----------+------+
| id | role_id | user_name | sex |
+----+---------+-----------+------+
| 1 |    1 | admin   |  1 |
| 2 |    2 | 王經(jīng)理  |  1 |
| 3 |    2 | 李經(jīng)理  |  2 |
| 4 |    2 | 張經(jīng)理  |  2 |
| 5 |    3 | 王科長  |  1 |
| 6 |    3 | 李科長  |  1 |
| 7 |    3 | 呂科長  |  2 |
| 8 |    3 | 邢科長  |  1 |
| 9 |    4 | 范組長  |  2 |
| 10 |    4 | 趙組長  |  2 |
| 11 |    4 | 姬組長  |  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 '性別'
  -> FROM
  -> USER;
+----+-----------+--------+
| id | 姓名   | 性別  |
+----+-----------+--------+
| 1 | admin   | 男   |
| 2 | 王經(jīng)理  | 男   |
| 3 | 李經(jīng)理  | 女   |
| 4 | 張經(jīng)理  | 女   |
| 5 | 王科長  | 男   |
| 6 | 李科長  | 男   |
| 7 | 呂科長  | 女   |
| 8 | 邢科長  | 男   |
| 9 | 范組長  | 女   |
| 10 | 趙組長  | 女   |
| 11 | 姬組長  | 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 | 管理員  |  0 |
| 2 | 總經(jīng)理  |  2 |
| 3 | 科長   |  1 |
| 4 | 組長   |  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 | 總經(jīng)理  |  2 |
| 3 | 科長   |  1 |
| 4 | 組長   |  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 = '總經(jīng)理'
  -> GROUP BY
  -> r.role_name
  -> ORDER BY
  -> r.id ASC;
+----+-----------+-----+
| id | role   | sex |
+----+-----------+-----+
| 2 | 總經(jīng)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 = '總經(jīng)理'
  -> GROUP BY
  -> r.role_name
  -> ORDER BY
  -> r.id ASC;
+----+-----------+-----+
| id | role   | sex |
+----+-----------+-----+
| 1 | 管理員  |  0 |
| 2 | 總經(jīng)理  |  3 |
| 3 | 科長   |  0 |
| 4 | 組長   |  0 |
+----+-----------+-----+
4 rows in set (0.00 sec)