MySQL联表查询基本操作之left-join常见的坑

概述

对于中小体量的项目而言，联表查询是再常见不过的操作了，尤其是在做报表的时候。然而校对数据的时候，您发现坑了吗？本篇文章就 mysql 常用联表查询复现常见的坑。

基础环境

建表语句

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)

简单信息报表: 查询用户信息

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)

找出角色为总经理的员工数量

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)

总结

在 left join 语句中，左表过滤必须放 where 条件中，右表过滤必须放 on 条件中，这样结果才能不多不少，刚刚好。

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