MySQL 多表查询（Day43）

Posted 2020-10-06 Neither Candidate

阅读目录

　　一，介绍

　　二，多表连接查询

　　三，符合条件链接查询

　　四，子查询

　　五，综合练习

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一，介绍

多表连接查询

复合条件连接查询

子查询

 

准备表，员工表和部门表

company.employee
company.department

#建表
create table department(
id int,
name varchar(20) 
);

create table employee1(
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 employee1(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 |
+----+------------+--------+------+--------+

 回到顶部

 

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，交叉链接：不适用任何匹配条件，生成笛卡尔积

 select * from employee,department;

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         |
+----+------------+--------+------+--------+------+--------------+

交叉连接查询

2，内连接：只连接匹配的行。（找到两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出正确的结果）

#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;

注意：内连接的join可以忽略不写，但是还是加上看起来清楚点

3，外连接之左连接：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，外连接之右连接：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：显示左右两个表的全部记录

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.id
union
select * 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

回到顶部

 

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,即找出公司所有部门中年龄大于25岁的员工
select employee.name,employee.age from employee,department
    where employee.dep_id = department.id
    and 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;

回到顶部

 

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：= 、 !=、> 、<等

1，带 in 关键字的子查询

#查询employee表，但dep_id必须在department表中出现过
select * from employee
    where dep_id in
        (select id from department);

2，带比较运算符的子查询

#比较运算符：=、!=、>、>=、<、<=、<>
#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);

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，Ture
mysql> 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，False
mysql> select * from employee
    ->     where exists
    ->         (select id from department where id=204);
Empty set (0.00 sec)

小练习：

#查询平均年龄在25岁以上的部门名
select 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 = (select id from department where name=\'技术\');

#查看小于2人的部门名
select name from department where id in (
select dep_id from employee1 group by dep_id having count(id) < 2
)
union
select name from department where id not in (select distinct dep_id from employee1);

#提取空部门                              #有人的部门
select * from department where id not in (select distinct dep_id from employee1);


或者：
select name from department where id in
(
select dep_id from employee1 group by dep_id having count(id) < 2
union
select id from department where id not in (select distinct dep_id from employee1);
);

回到顶部

 

五，综合练习

1，select语句关键字的定义顺序

select distinct <select_list>
from <left_table>
<join_type> join <right_table>
on <join_condition>
where <where_condition>
group by <group_by_list>
having <having_condition>
order by<order_by_condition>
limit<limit_number>

select语句关键字的定义顺序

 

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\', \'
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