概述

對(duì)于中小體量的項(xiàng)目而言，聯(lián)表查詢是再常見(jiàn)不過(guò)的操作了，尤其是在做報(bào)表的時(shí)候。然而校對(duì)數(shù)據(jù)的時(shí)候，您發(fā)現(xiàn)坑了嗎？本篇文章就 mysql 常用聯(lián)表查詢復(fù)現(xiàn)常見(jiàn)的坑。

基礎(chǔ)環(huán)境

建表語(yǔ)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, ’管理員’);insert into `role` VALUES(2, ’總經(jīng)理’);insert into `role` VALUES(3, ’科長(zhǎng)’);insert into `role` VALUES(4, ’組長(zhǎng)’);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(zhǎng)’, 1);insert into `user` VALUES(6, 3, ’李科長(zhǎng)’, 1);insert into `user` VALUES(7, 3, ’呂科長(zhǎng)’, 2);insert into `user` VALUES(8, 3, ’邢科長(zhǎng)’, 1);insert into `user` VALUES(9, 4, ’范組長(zhǎng)’, 2);insert into `user` VALUES(10, 4, ’趙組長(zhǎng)’, 2);insert into `user` VALUES(11, 4, ’姬組長(zhǎng)’, 1);

數(shù)據(jù)如下

mysql> select * from role;+----+-----------+| id | role_name |+----+-----------+| 1 | 管理員 || 2 | 總經(jīng)理 || 3 | 科長(zhǎng) || 4 | 組長(zhǎng) |+----+-----------+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(zhǎng) | 1 || 6 | 3 | 李科長(zhǎng) | 1 || 7 | 3 | 呂科長(zhǎng) | 2 || 8 | 3 | 邢科長(zhǎng) | 1 || 9 | 4 | 范組長(zhǎng) | 2 || 10 | 4 | 趙組長(zhǎng) | 2 || 11 | 4 | 姬組長(zhǎng) | 1 |+----+---------+-----------+------+11 rows in set (0.00 sec)

基本業(yè)務(wù)

簡(jiǎn)單信息報(bào)表: 查詢用戶信息

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(zhǎng) | 男 || 6 | 李科長(zhǎng) | 男 || 7 | 呂科長(zhǎng) | 女 || 8 | 邢科長(zhǎng) | 男 || 9 | 范組長(zhǎng) | 女 || 10 | 趙組長(zhǎng) | 女 || 11 | 姬組長(zhǎng) | 男 |+----+-----------+--------+

查詢每個(gè)角色名稱及對(duì)應(yīng)人員中女性數(shù)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 -> 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(zhǎng) | 1 || 4 | 組長(zhǎng) | 2 |+----+-----------+-----+4 rows in set (0.00 sec)

假如我們把性別過(guò)濾的條件改為 where 操作結(jié)果會(huì)怎么樣呢？

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(zhǎng) | 1 || 4 | 組長(zhǎng) | 2 |+----+-----------+-----+3 rows in set (0.00 sec)

這里可以看到角色數(shù)據(jù)不完整了。

找出角色為總經(jīng)理的員工數(shù)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 = ’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)

同樣將過(guò)濾條件由 where 改為 on

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(zhǎng) | 0 || 4 | 組長(zhǎng) | 0 |+----+-----------+-----+4 rows in set (0.00 sec)

這里可以看到數(shù)據(jù)多余了

總結(jié)

在 left join 語(yǔ)句中，左表過(guò)濾必須放 where 條件中，右表過(guò)濾必須放 on 條件中，這樣結(jié)果才能不多不少，剛剛好。

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