6.多表查询

Wesley13
• 阅读 376

多表查询

本节重点:

  • 多表连接查询
  • 符合条件连接查询
  • 子查询

准备工作:准备两张表,部门表(department)、员工表(employee)

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),
('nvshen','male',18,200),
('xiaomage','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    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.19 sec)

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    |                |
+--------+-----------------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

mysql> select * from department;
+------+--------------+
| id   | name         |
+------+--------------+
|  200 | 技术         |
|  201 | 人力资源     |
|  202 | 销售         |
|  203 | 运营         |
+------+--------------+
4 rows in set (0.02 sec)

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 | nvshen   | male   |   18 |    200 |
|  6 | xiaomage | female |   18 |    204 |
+----+----------+--------+------+--------+
6 rows in set (0.00 sec)

ps:观察两张表,发现department表中id=203部门在employee中没有对应的员工,发现employee中id=6的员工在department表中没有对应关系。

一、多表连接查询

两张表的准备工作已完成,比如现在我要查询的员工信息以及该员工所在的部门。从该题中,我们看出既要查员工又要查该员工的部门,肯定要将两张表进行连接查询,多表连接查询。

重点:外链接语法

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)先看第一种情况交叉连接:不适用任何匹配条件。生成笛卡尔积(关于笛卡尔积的含义,大家百度自行补脑)。

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 | nvshen   | male   |   18 |    200 |  200 | 技术         |
|  5 | nvshen   | male   |   18 |    200 |  201 | 人力资源     |
|  5 | nvshen   | male   |   18 |    200 |  202 | 销售         |
|  5 | nvshen   | male   |   18 |    200 |  203 | 运营         |
|  6 | xiaomage | female |   18 |    204 |  200 | 技术         |
|  6 | xiaomage | female |   18 |    204 |  201 | 人力资源     |
|  6 | xiaomage | female |   18 |    204 |  202 | 销售         |
|  6 | xiaomage | 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匹配的结果
#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 | nvshen  |   18 | male   | 技术         |
+----+---------+------+--------+--------------+
5 rows in set (0.00 sec)

#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 | nvshen   | 技术         |
|  2 | alex     | 人力资源     |
|  3 | wupeiqi  | 人力资源     |
|  4 | yuanhao  | 销售         |
|  6 | xiaomage | NULL         |
+----+----------+--------------+
6 rows in set (0.00 sec)

(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 | nvshen  | 技术         |
| NULL | NULL    | 运营         |
+------+---------+--------------+
6 rows in set (0.00 sec)

(5) 全外连接:显示左右两个表全部记录(了解)

#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
语法:select * from employee left join department on employee.dep_id = department.id 
       union all
      select * from employee right join department on employee.dep_id = department.id;

 mysql> 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
           ;
+------+----------+--------+------+--------+------+--------------+
| id   | name     | sex    | age  | dep_id | id   | name         |
+------+----------+--------+------+--------+------+--------------+
|    1 | egon     | male   |   18 |    200 |  200 | 技术         |
|    5 | nvshen   | 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 | xiaomage | female |   18 |    204 | NULL | NULL         |
| NULL | NULL     | NULL   | NULL |   NULL |  203 | 运营         |
+------+----------+--------+------+--------+------+--------------+
7 rows in set (0.01 sec)

#注意 union与union all的区别:union会去掉相同的纪录

二、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;

三、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

6.多表查询 6.多表查询

子查询(嵌套查询): 查多次, 多个select

注意: 第一次的查询结果可以作为第二次的查询的 条件 或者 表名 使用.

子查询中可以包含:IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字. 还可以包含比较运算符:= 、 !=、> 、<等.

 1.作为表名使用
select * from (select * from person) as 表名;
 
ps:大家需要注意的是: 一条语句中可以有多个这样的子查询,在执行时,最里层括号(sql语句) 具有优先执行权.<br>注意: as 后面的表名称不能加引号('')
 2.求最大工资那个人的姓名和薪水

复制代码
1.求最大工资
select max(salary) from person;
2.求最大工资那个人叫什么
select name,salary from person where salary=53000;

合并
select name,salary from person where salary=(select max(salary) from person);
复制代码
复制代码
3. 求工资高于所有人员平均工资的人员

复制代码
1.求平均工资
select avg(salary) from person;

2.工资大于平均工资的 人的姓名、工资
select name,salary from person where salary > 21298.625;

合并
select name,salary from person where salary >(select avg(salary) from person);
复制代码

子查询示例

例子:

(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 not in 
        (select dep_id from employee group by dep_id);

(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 employee where age > (select avg(age) from employee);
+---------+------+
| name    | age  |
+---------+------+
| alex    |   48 |
| wupeiqi |   38 |
+---------+------+

#查询大于部门内平均年龄的员工名、年龄
思路:
      (1)先对员工表(employee)中的人员分组(group by),查询出dep_id以及平均年龄。       (2)将查出的结果作为临时表,再对根据临时表的dep_id和employee的dep_id作为筛选条件将employee表和临时表进行内连接。
       (3)最后再将employee员工的年龄是大于平均年龄的员工名字和年龄筛选。



mysql> select t1.name,t1.age from employee as t1
             inner join
            (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2
            on t1.dep_id = t2.dep_id
            where t1.age > t2.avg_age;
+------+------+
| name | age  |
+------+------+
| alex |   48 |

(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 | nvshen   | male   |   18 |    200 |
|  6 | xiaomage | 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:

查询每个部门最新入职的那位员工

#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','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)
点赞
收藏
评论区
推荐文章
blmius blmius
2年前
MySQL:[Err] 1292 - Incorrect datetime value: ‘0000-00-00 00:00:00‘ for column ‘CREATE_TIME‘ at row 1
文章目录问题用navicat导入数据时,报错:原因这是因为当前的MySQL不支持datetime为0的情况。解决修改sql\mode:sql\mode:SQLMode定义了MySQL应支持的SQL语法、数据校验等,这样可以更容易地在不同的环境中使用MySQL。全局s
Easter79 Easter79
2年前
sql多表连接查询
本文主要列举两张和三张表来讲述多表连接查询。新建两张表:表1:student 截图如下:!SQL多表连接查询(详细实例)_新客网(http://static.oschina.net/uploads/img/201406/12160627_gvI7.jpg)表2:course 截图如下:!SQL多表连接查询(详细实例)_
Wesley13 Wesley13
2年前
9 MySQL
多表查询:http://www.cnblogs.com/linhaifeng/articles/7267596.html1.多表连接查询2.符合条件连接查询3.子查询一、准备表!(https://oscimg.oschina.net/oscnet/3ed677
Wesley13 Wesley13
2年前
Oracle——分页查询
查询员工表中,工资排名在1020之间的员工信息。select  from(    selectrownumrn,employee_id,salary      from(        selectemployee_id,salary,last_name        fromem
Wesley13 Wesley13
2年前
mysql5.6 分页查询优化
mysql5.6分页查询优化场景:表结构:主键(非自增)contentCode(varchar),过滤条件列为updateTime(timeStamp),已经为timestamp建立索引。搜索sql为:SELECTFROMmy_hello_tableWHEREupdat
Wesley13 Wesley13
2年前
oracle查询表数据并重新插入到本表
oracle查询表数据并重新插入到本表CreateTime2018年5月17日10:30:10Author:Marydon1.情景描述查询表中数据SELECTFROMat_aut
Wesley13 Wesley13
2年前
ThinkPHP 根据关联数据查询 hasWhere 的使用实例
很多时候,模型关联后需要根据关联的模型做查询。场景:广告表(ad),广告类型表(ad\_type),现在需要筛选出广告类型表中id字段为1且广告表中status为1的列表先看关联的设置部分 publicfunctionadType(){return$thisbelongsTo('A
Wesley13 Wesley13
2年前
MySQL之多表查询
阅读目录一多表联合查询二多表连接查询三复杂条件多表查询四子语句查询五其他方式查询六 SQL逻辑查询语句执行顺序(重点)七外键约束八其他约束类型九表与表之间的关系一.多表联合查询!
Wesley13 Wesley13
2年前
mysql出现Waiting for table metadata lock的解决方法
查询某一个表时,一直没有显示数据,于是就showprocesslist;!(https://oscimg.oschina.net/oscnet/07f86cfca979bba85837f1b6352eb00ec33.jpg)发现有表已经被锁了,关掉了之前的查询语句可以看到!(https://oscimg.oschina.net/oscn
为什么mysql不推荐使用雪花ID作为主键
作者:毛辰飞背景在mysql中设计表的时候,mysql官方推荐不要使用uuid或者不连续不重复的雪花id(long形且唯一),而是推荐连续自增的主键id,官方的推荐是auto_increment,那么为什么不建议采用uuid,使用uuid究