mysql多表查询-白红宇

mysql多表查询

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发布时间：2019-06-20

本文共 19801 字，大约阅读时间需要 66 分钟。

本节主题

多表连接查询

复合条件连接查询

子查询

准备表

#建表create table department(id int,name varchar(20) );create table employee(id int primary key auto_increment,name varchar(20),sex enum('male','female') not null default 'male',age int,dep_id int);#插入数据insert into department values(200,'技术'),(201,'人力资源'),(202,'销售'),(203,'运营');insert into employee(name,sex,age,dep_id) values('egon','male',18,200),('alex','female',48,201),('wupeiqi','male',38,201),('yuanhao','female',28,202),('liwenzhou','male',18,200),('jingliyang','female',18,204);#查看表结构和数据mysql> desc department;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(11) | YES | | NULL | || name | varchar(20) | YES | | NULL | |+-------+-------------+------+-----+---------+-------+mysql> desc employee;+--------+-----------------------+------+-----+---------+----------------+| Field | Type | Null | Key | Default | Extra |+--------+-----------------------+------+-----+---------+----------------+| id | int(11) | NO | PRI | NULL | auto_increment || name | varchar(20) | YES | | NULL | || sex | enum('male','female') | NO | | male | || age | int(11) | YES | | NULL | || dep_id | int(11) | YES | | NULL | |+--------+-----------------------+------+-----+---------+----------------+mysql> select * from department;+------+--------------+| id | name |+------+--------------+| 200 | 技术 || 201 | 人力资源 || 202 | 销售 || 203 | 运营 |+------+--------------+mysql> select * from employee;+----+------------+--------+------+--------+| id | name | sex | age | dep_id |+----+------------+--------+------+--------+| 1 | egon | male | 18 | 200 || 2 | alex | female | 48 | 201 || 3 | wupeiqi | male | 38 | 201 || 4 | yuanhao | female | 28 | 202 || 5 | liwenzhou | male | 18 | 200 || 6 | jingliyang | female | 18 | 204 |+----+------------+--------+------+--------+表department与employee

二多表连接查询

#重点：外链接语法SELECT 字段列表    FROM 表1 INNER|LEFT|RIGHT JOIN 表2    ON 表1.字段 = 表2.字段;

1 交叉连接：不适用任何匹配条件。生成笛卡尔积

mysql> select * from employee,department;+----+------------+--------+------+--------+------+--------------+| id | name       | sex    | age  | dep_id | id   | name         |+----+------------+--------+------+--------+------+--------------+|  1 | egon       | male   |   18 |    200 |  200 | 技术         ||  1 | egon       | male   |   18 |    200 |  201 | 人力资源     ||  1 | egon       | male   |   18 |    200 |  202 | 销售         ||  1 | egon       | male   |   18 |    200 |  203 | 运营         ||  2 | alex       | female |   48 |    201 |  200 | 技术         ||  2 | alex       | female |   48 |    201 |  201 | 人力资源     ||  2 | alex       | female |   48 |    201 |  202 | 销售         ||  2 | alex       | female |   48 |    201 |  203 | 运营         ||  3 | wupeiqi    | male   |   38 |    201 |  200 | 技术         ||  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     ||  3 | wupeiqi    | male   |   38 |    201 |  202 | 销售         ||  3 | wupeiqi    | male   |   38 |    201 |  203 | 运营         ||  4 | yuanhao    | female |   28 |    202 |  200 | 技术         ||  4 | yuanhao    | female |   28 |    202 |  201 | 人力资源     ||  4 | yuanhao    | female |   28 |    202 |  202 | 销售         ||  4 | yuanhao    | female |   28 |    202 |  203 | 运营         ||  5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         ||  5 | liwenzhou  | male   |   18 |    200 |  201 | 人力资源     ||  5 | liwenzhou  | male   |   18 |    200 |  202 | 销售         ||  5 | liwenzhou  | male   |   18 |    200 |  203 | 运营         ||  6 | jingliyang | female |   18 |    204 |  200 | 技术         ||  6 | jingliyang | female |   18 |    204 |  201 | 人力资源     ||  6 | jingliyang | female |   18 |    204 |  202 | 销售         ||  6 | jingliyang | female |   18 |    204 |  203 | 运营         |+----+------------+--------+------+--------+------+--------------+

2 内连接：只连接匹配的行

#找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果#department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id; +----+-----------+------+--------+--------------+| id | name      | age  | sex    | name         |+----+-----------+------+--------+--------------+|  1 | egon      |   18 | male   | 技术         ||  2 | alex      |   48 | female | 人力资源     ||  3 | wupeiqi   |   38 | male   | 人力资源     ||  4 | yuanhao   |   28 | female | 销售         ||  5 | liwenzhou |   18 | male   | 技术         |+----+-----------+------+--------+--------------+#上述sql等同于mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

3 外链接之左连接：优先显示左表全部记录

#以左表为准，即找出所有员工信息，当然包括没有部门的员工#本质就是：在内连接的基础上增加左边有右边没有的结果mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;+----+------------+--------------+| id | name       | depart_name  |+----+------------+--------------+|  1 | egon       | 技术         ||  5 | liwenzhou  | 技术         ||  2 | alex       | 人力资源     ||  3 | wupeiqi    | 人力资源     ||  4 | yuanhao    | 销售         ||  6 | jingliyang | NULL         |+----+------------+--------------+

4 外链接之右连接：优先显示右表全部记录

#以右表为准，即找出所有部门信息，包括没有员工的部门#本质就是：在内连接的基础上增加右边有左边没有的结果mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;+------+-----------+--------------+| id   | name      | depart_name  |+------+-----------+--------------+|    1 | egon      | 技术         ||    2 | alex      | 人力资源     ||    3 | wupeiqi   | 人力资源     ||    4 | yuanhao   | 销售         ||    5 | liwenzhou | 技术         || NULL | NULL      | 运营         |+------+-----------+--------------+

5 全外连接：显示左右两个表全部记录

全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果#注意：mysql不支持全外连接 full JOIN#强调：mysql可以使用此种方式间接实现全外连接select * from employee left join department on employee.dep_id = department.idunionselect * from employee right join department on employee.dep_id = department.id;#查看结果+------+------------+--------+------+--------+------+--------------+| id   | name       | sex    | age  | dep_id | id   | name         |+------+------------+--------+------+--------+------+--------------+|    1 | egon       | male   |   18 |    200 |  200 | 技术         ||    5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         ||    2 | alex       | female |   48 |    201 |  201 | 人力资源     ||    3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     ||    4 | yuanhao    | female |   28 |    202 |  202 | 销售         ||    6 | jingliyang | female |   18 |    204 | NULL | NULL         || NULL | NULL       | NULL   | NULL |   NULL |  203 | 运营         |+------+------------+--------+------+--------+------+--------------+#注意 union与union all的区别：union会去掉相同的纪录

三符合条件连接查询

#示例1：以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门select employee.name,department.name from employee inner join department    on employee.dep_id = department.id    where age > 25;#示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示select employee.id,employee.name,employee.age,department.name from employee,department    where employee.dep_id = department.id    and age > 25    order by age asc;

四子查询

#1：子查询是将一个查询语句嵌套在另一个查询语句中。#2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。#3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字#4：还可以包含比较运算符：= 、 !=、> 、

1 带IN关键字的子查询

#查询平均年龄在25岁以上的部门名select id,name from department    where id in         (select dep_id from employee group by dep_id having avg(age) > 25);#查看技术部员工姓名select name from employee    where dep_id in         (select id from department where name='技术');#查看不足1人的部门名select name from department    where id in         (select dep_id from employee group by dep_id having count(id) <=1);

2 带比较运算符的子查询

#比较运算符：=、!=、>、>=、<、<=、<>#查询大于所有人平均年龄的员工名与年龄mysql> select name,age from emp where age > (select avg(age) from emp);+---------+------+| name | age |+---------+------+| alex | 48 || wupeiqi | 38 |+---------+------+rows in set (0.00 sec)#查询大于部门内平均年龄的员工名、年龄select t1.name,t1.age from emp t1inner join (select dep_id,avg(age) avg_age from emp group by dep_id) t2on t1.dep_id = t2.dep_idwhere t1.age > t2.avg_age;

3 带EXISTS关键字的子查询

EXISTS关字键字表示存在。在使用EXISTS关键字时，内层查询语句不返回查询的记录。

而是返回一个真假值。True或False

当返回True时，外层查询语句将进行查询；当返回值为False时，外层查询语句不进行查询

#department表中存在dept_id=203，Turemysql> select * from employee    ->     where exists    ->         (select id from department where id=200);+----+------------+--------+------+--------+| id | name       | sex    | age  | dep_id |+----+------------+--------+------+--------+|  1 | egon       | male   |   18 |    200 ||  2 | alex       | female |   48 |    201 ||  3 | wupeiqi    | male   |   38 |    201 ||  4 | yuanhao    | female |   28 |    202 ||  5 | liwenzhou  | male   |   18 |    200 ||  6 | jingliyang | female |   18 |    204 |+----+------------+--------+------+--------+#department表中存在dept_id=205，Falsemysql> select * from employee    ->     where exists    ->         (select id from department where id=204);Empty set (0.00 sec)

练习：查询每个部门最新入职的那位员工

company.employee    员工id      id                  int                 姓名        emp_name            varchar    性别        sex                 enum    年龄        age                 int    入职日期     hire_date           date    岗位        post                varchar    职位描述     post_comment        varchar    薪水        salary              double    办公室       office              int    部门编号     depart_id           int#创建表create table employee(id int not null unique auto_increment,name varchar(20) not null,sex enum('male','female') not null default 'male', #大部分是男的age int(3) unsigned not null default 28,hire_date date not null,post varchar(50),post_comment varchar(100),salary double(15,2),office int, #一个部门一个屋子depart_id int);#查看表结构mysql> desc employee;+--------------+-----------------------+------+-----+---------+----------------+| Field        | Type                  | Null | Key | Default | Extra          |+--------------+-----------------------+------+-----+---------+----------------+| id           | int(11)               | NO   | PRI | NULL    | auto_increment || name         | varchar(20)           | NO   |     | NULL    |                || sex          | enum('male','female') | NO   |     | male    |                || age          | int(3) unsigned       | NO   |     | 28      |                || hire_date    | date                  | NO   |     | NULL    |                || post         | varchar(50)           | YES  |     | NULL    |                || post_comment | varchar(100)          | YES  |     | NULL    |                || salary       | double(15,2)          | YES  |     | NULL    |                || office       | int(11)               | YES  |     | NULL    |                || depart_id    | int(11)               | YES  |     | NULL    |                |+--------------+-----------------------+------+-----+---------+----------------+#插入记录#三个部门：教学，销售，运营insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部('alex','male',78,'20150302','teacher',1000000.31,401,1),('wupeiqi','male',81,'20130305','teacher',8300,401,1),('yuanhao','male',73,'20140701','teacher',3500,401,1),('liwenzhou','male',28,'20121101','teacher',2100,401,1),('jingliyang','female',18,'20110211','teacher',9000,401,1),('jinxin','male',18,'19000301','teacher',30000,401,1),('成龙','male',48,'20101111','teacher',10000,401,1),('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门('丫丫','female',38,'20101101','sale',2000.35,402,2),('丁丁','female',18,'20110312','sale',1000.37,402,2),('星星','female',18,'20160513','sale',3000.29,402,2),('格格','female',28,'20170127','sale',4000.33,402,2),('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门('程咬金','male',18,'19970312','operation',20000,403,3),('程咬银','female',18,'20130311','operation',19000,403,3),('程咬铜','male',18,'20150411','operation',18000,403,3),('程咬铁','female',18,'20140512','operation',17000,403,3);#ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk准备表和记录

SELECT    *FROM    emp AS t1INNER JOIN (    SELECT        post,        max(hire_date) max_date    FROM        emp    GROUP BY        post) AS t2 ON t1.post = t2.postWHERE    t1.hire_date = t2.max_date;答案一（链表）

mysql> select (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;+---------------------------------------------------------------------------------------+| (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |+---------------------------------------------------------------------------------------+| 张野                                                                                  || 格格                                                                                  || alex                                                                                  || egon                                                                                  |+---------------------------------------------------------------------------------------+rows in set (0.00 sec)mysql> select (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;+-------------------------------------------------------------------------------------+| (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |+-------------------------------------------------------------------------------------+|                                                                                  14 ||                                                                                  13 ||                                                                                   2 ||                                                                                   1 |+-------------------------------------------------------------------------------------+rows in set (0.00 sec)#正确答案mysql> select t3.name,t3.post,t3.hire_date from emp as t3 where id in (select (select id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post);+--------+-----------------------------------------+------------+| name   | post                                    | hire_date  |+--------+-----------------------------------------+------------+| egon   | 老男孩驻沙河办事处外交大使              | 2017-03-01 || alex   | teacher                                 | 2015-03-02 || 格格   | sale                                    | 2017-01-27 || 张野   | operation                               | 2016-03-11 |+--------+-----------------------------------------+------------+rows in set (0.00 sec)答案二（子查询）

答案一为正确答案，答案二中的limit 1有问题（每个部门可能有>1个为同一时间入职的新员工），我只是想用该例子来说明可以在select后使用子查询

可以基于上述方法解决：比如某网站在全国各个市都有站点，每个站点一条数据，想取每个省下最新的那一条市的网站质量信息

五综合练习

init.sql文件内容

/* 数据导入： Navicat Premium Data Transfer Source Server         : localhost Source Server Type    : MySQL Source Server Version : 50624 Source Host           : localhost Source Database       : sqlexam Target Server Type    : MySQL Target Server Version : 50624 File Encoding         : utf-8 Date: 10/21/2016 06:46:46 AM*/SET NAMES utf8;SET FOREIGN_KEY_CHECKS = 0;-- ------------------------------  Table structure for `class`-- ----------------------------DROP TABLE IF EXISTS `class`;CREATE TABLE `class` (  `cid` int(11) NOT NULL AUTO_INCREMENT,  `caption` varchar(32) NOT NULL,  PRIMARY KEY (`cid`)) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;-- ------------------------------  Records of `class`-- ----------------------------BEGIN;INSERT INTO `class` VALUES ('1', '三年二班'), ('2', '三年三班'), ('3', '一年二班'), ('4', '二年九班');COMMIT;-- ------------------------------  Table structure for `course`-- ----------------------------DROP TABLE IF EXISTS `course`;CREATE TABLE `course` (  `cid` int(11) NOT NULL AUTO_INCREMENT,  `cname` varchar(32) NOT NULL,  `teacher_id` int(11) NOT NULL,  PRIMARY KEY (`cid`),  KEY `fk_course_teacher` (`teacher_id`),  CONSTRAINT `fk_course_teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`tid`)) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;-- ------------------------------  Records of `course`-- ----------------------------BEGIN;INSERT INTO `course` VALUES ('1', '生物', '1'), ('2', '物理', '2'), ('3', '体育', '3'), ('4', '美术', '2');COMMIT;-- ------------------------------  Table structure for `score`-- ----------------------------DROP TABLE IF EXISTS `score`;CREATE TABLE `score` (  `sid` int(11) NOT NULL AUTO_INCREMENT,  `student_id` int(11) NOT NULL,  `course_id` int(11) NOT NULL,  `num` int(11) NOT NULL,  PRIMARY KEY (`sid`),  KEY `fk_score_student` (`student_id`),  KEY `fk_score_course` (`course_id`),  CONSTRAINT `fk_score_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`cid`),  CONSTRAINT `fk_score_student` FOREIGN KEY (`student_id`) REFERENCES `student` (`sid`)) ENGINE=InnoDB AUTO_INCREMENT=53 DEFAULT CHARSET=utf8;-- ------------------------------  Records of `score`-- ----------------------------BEGIN;INSERT INTO `score` VALUES ('1', '1', '1', '10'), ('2', '1', '2', '9'), ('5', '1', '4', '66'), ('6', '2', '1', '8'), ('8', '2', '3', '68'), ('9', '2', '4', '99'), ('10', '3', '1', '77'), ('11', '3', '2', '66'), ('12', '3', '3', '87'), ('13', '3', '4', '99'), ('14', '4', '1', '79'), ('15', '4', '2', '11'), ('16', '4', '3', '67'), ('17', '4', '4', '100'), ('18', '5', '1', '79'), ('19', '5', '2', '11'), ('20', '5', '3', '67'), ('21', '5', '4', '100'), ('22', '6', '1', '9'), ('23', '6', '2', '100'), ('24', '6', '3', '67'), ('25', '6', '4', '100'), ('26', '7', '1', '9'), ('27', '7', '2', '100'), ('28', '7', '3', '67'), ('29', '7', '4', '88'), ('30', '8', '1', '9'), ('31', '8', '2', '100'), ('32', '8', '3', '67'), ('33', '8', '4', '88'), ('34', '9', '1', '91'), ('35', '9', '2', '88'), ('36', '9', '3', '67'), ('37', '9', '4', '22'), ('38', '10', '1', '90'), ('39', '10', '2', '77'), ('40', '10', '3', '43'), ('41', '10', '4', '87'), ('42', '11', '1', '90'), ('43', '11', '2', '77'), ('44', '11', '3', '43'), ('45', '11', '4', '87'), ('46', '12', '1', '90'), ('47', '12', '2', '77'), ('48', '12', '3', '43'), ('49', '12', '4', '87'), ('52', '13', '3', '87');COMMIT;-- ------------------------------  Table structure for `student`-- ----------------------------DROP TABLE IF EXISTS `student`;CREATE TABLE `student` (  `sid` int(11) NOT NULL AUTO_INCREMENT,  `gender` char(1) NOT NULL,  `class_id` int(11) NOT NULL,  `sname` varchar(32) NOT NULL,  PRIMARY KEY (`sid`),  KEY `fk_class` (`class_id`),  CONSTRAINT `fk_class` FOREIGN KEY (`class_id`) REFERENCES `class` (`cid`)) ENGINE=InnoDB AUTO_INCREMENT=17 DEFAULT CHARSET=utf8;-- ------------------------------  Records of `student`-- ----------------------------BEGIN;INSERT INTO `student` VALUES ('1', '男', '1', '理解'), ('2', '女', '1', '钢蛋'), ('3', '男', '1', '张三'), ('4', '男', '1', '张一'), ('5', '女', '1', '张二'), ('6', '男', '1', '张四'), ('7', '女', '2', '铁锤'), ('8', '男', '2', '李三'), ('9', '男', '2', '李一'), ('10', '女', '2', '李二'), ('11', '男', '2', '李四'), ('12', '女', '3', '如花'), ('13', '男', '3', '刘三'), ('14', '男', '3', '刘一'), ('15', '女', '3', '刘二'), ('16', '男', '3', '刘四');COMMIT;-- ------------------------------  Table structure for `teacher`-- ----------------------------DROP TABLE IF EXISTS `teacher`;CREATE TABLE `teacher` (  `tid` int(11) NOT NULL AUTO_INCREMENT,  `tname` varchar(32) NOT NULL,  PRIMARY KEY (`tid`)) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;-- ------------------------------  Records of `teacher`-- ----------------------------BEGIN;INSERT INTO `teacher` VALUES ('1', '张磊老师'), ('2', '李平老师'), ('3', '刘海燕老师'), ('4', '朱云海老师'), ('5', '李杰老师');COMMIT;SET FOREIGN_KEY_CHECKS = 1;

转载于:https://www.cnblogs.com/fmgao-technology/p/9208712.html

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