一、外键的变种(三种关系)

本节重点:

  • 如何找出两张表之间的关系

  • 表的三种关系

一、介绍

因为有foreign key的约束,使得两张表形成了三种了关系:

  • 多对一
  • 多对多
  • 一对一

二、重点理解如果找出两张表之间的关系

分析步骤:

#1、先站在左表的角度去找
是否左表的多条记录可以对应右表的一条记录,如果是,则证明左表的一个字段foreign key 右表一个字段(通常是id) #2、再站在右表的角度去找
是否右表的多条记录可以对应左表的一条记录,如果是,则证明右表的一个字段foreign key 左表一个字段(通常是id) #3、总结:
#多对一:
如果只有步骤1成立,则是左表多对一右表
如果只有步骤2成立,则是右表多对一左表 #多对多
如果步骤1和2同时成立,则证明这两张表时一个双向的多对一,即多对多,需要定义一个这两张表的关系表来专门存放二者的关系 #一对一:
如果1和2都不成立,而是左表的一条记录唯一对应右表的一条记录,反之亦然。这种情况很简单,就是在左表foreign key右表的基础上,将左表的外键字段设置成unique即可

三、表的三种关系

(1)书和出版社

  一对多(或多对一):一个出版社可以出版多本书。看图说话。

  关联方式:foreign key

一本书,对应一个出版社,是正常的。

一本书,对应多个出版社,那就是盗版的。

多本书,可以对应一个出版社,比如工业出版社。

总结:

左表的多条记录可以对应右边的一条记录,则左表的一个字段foreign key 右表一个字段(通常是id)

书和出版社(多对一)

create table press(
id int primary key auto_increment,
name varchar(20)
); create table book(
id int primary key auto_increment,
name varchar(20),
press_id int not null,
constraint fk_book_press foreign key(press_id) references press(id)
on delete cascade
on update cascade
); # 先往被关联表中插入记录
insert into press(name) values
('北京工业地雷出版社'),
('人民音乐不好听出版社'),
('知识产权没有用出版社')
; # 再往关联表中插入记录
insert into book(name,press_id) values
('九阳神功',1),
('九阴真经',2),
('九阴白骨爪',2),
('独孤九剑',3),
('降龙十巴掌',2),
('葵花宝典',3)
; 查询结果:
mysql> select * from book;
+----+-----------------+----------+
| id | name | press_id |
+----+-----------------+----------+
| 1 | 九阳神功 | 1 |
| 2 | 九阴真经 | 2 |
| 3 | 九阴白骨爪 | 2 |
| 4 | 独孤九剑 | 3 |
| 5 | 降龙十巴掌 | 2 |
| 6 | 葵花宝典 | 3 |
+----+-----------------+----------+
rows in set (0.00 sec) mysql> select * from press;
+----+--------------------------------+
| id | name |
+----+--------------------------------+
| 1 | 北京工业地雷出版社 |
| 2 | 人民音乐不好听出版社 |
| 3 | 知识产权没有用出版社 |
+----+--------------------------------+
rows in set (0.00 sec) 书和出版社(多对一)

(2)作者和书籍的关系

  多对多:一个作者可以写多本书,一本书也可以有多个作者,双向的一对多,即多对多看图说话。

看上图,egon写了2本书,那么如何表示他写了2本书呢?

新增一个字段book_id,填写1,3。它对应的是右边表的id。

一本书,也可以有多个作者,新增字段author_id,填写1,2。它对应的是左边的id。

由于2个表,相互关联,不知道先创建谁?

解决方案,看下图:

  关联方式:foreign key+一张新的表

作者与书籍关系(多对多)

# 创建被关联表author表,之前的book表在讲多对一的关系已创建
create table author(
id int primary key auto_increment,
name varchar(20)
);
#这张表就存放了author表和book表的关系,即查询二者的关系查这表就可以了
create table author2book(
id int not null unique auto_increment,
author_id int not null,
book_id int not null,
constraint fk_author foreign key(author_id) references author(id)
on delete cascade
on update cascade,
constraint fk_book foreign key(book_id) references book(id)
on delete cascade
on update cascade,
primary key(author_id,book_id)
);
#插入四个作者,id依次排开
insert into author(name) values('egon'),('alex'),('wusir'),('yuanhao'); # 每个作者的代表作
egon: 九阳神功、九阴真经、九阴白骨爪、独孤九剑、降龙十巴掌、葵花宝典
alex: 九阳神功、葵花宝典
wusir:独孤九剑、降龙十巴掌、葵花宝典
yuanhao:九阳神功 # 在author2book表中插入相应的数据 insert into author2book(author_id,book_id) values
(1,1),
(1,2),
(1,3),
(1,4),
(1,5),
(1,6),
(2,1),
(2,6),
(3,4),
(3,5),
(3,6),
(4,1)
;
# 现在就可以查author2book对应的作者和书的关系了
mysql> select * from author2book;
+----+-----------+---------+
| id | author_id | book_id |
+----+-----------+---------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 1 | 4 |
| 5 | 1 | 5 |
| 6 | 1 | 6 |
| 7 | 2 | 1 |
| 8 | 2 | 6 |
| 9 | 3 | 4 |
| 10 | 3 | 5 |
| 11 | 3 | 6 |
| 12 | 4 | 1 |
+----+-----------+---------+
rows in set (0.00 sec) 作者与书籍关系(多对多)

总结:

多对多:
先建多个主表,再通过第三张表(从表)将多个主表通过外键关系进行关联。

(3)用户和博客

  一对一:一个用户只能注册一个博客,即一对一的关系。看图说话

  关联方式:foreign key+unique

用户和博客(一对一)

#例如: 一个用户只能注册一个博客

#两张表: 用户表 (user)和 博客表(blog)
# 创建用户表
create table user(
id int primary key auto_increment,
name varchar(20)
);
# 创建博客表,user_id 字段必须唯一
create table blog(
id int primary key auto_increment,
url varchar(100),
user_id int unique,
constraint fk_user foreign key(user_id) references user(id)
on delete cascade
on update cascade
);
#插入用户表中的记录
insert into user(name) values
('alex'),
('wusir'),
('egon'),
('xiaoma')
;
# 插入博客表的记录
insert into blog(url,user_id) values
('http://www.cnblog/alex',1),
('http://www.cnblog/wusir',2),
('http://www.cnblog/egon',3),
('http://www.cnblog/xiaoma',4)
;
# 查询wusir的博客地址
select url from blog where user_id=2;

总结:

在公司,一般不让用外键,影响性能,它会消耗硬盘。
下面的内容,将会介绍多表查询,来解决这个问题。

二、数据的增删改

本节重点:

  • 插入数据 INSERT

  • 更新数据 UPDATE

  • 删除数据 DELETE

再来回顾一下之前我们练过的一些操作,相信大家都对插入数据、更新数据、删除数据有了全面的认识。那么在mysql中其实最重要的不是这三大操作,而是查数据最重要,下节课知晓

一、在MySQL管理软件中,通过SQL语句中的DML语言来实现数据的操作

1.使用INSERT实现数据的插入
2.UPDATE实现数据的更新
3.使用DELETE实现数据的删除
4.使用SELECT查询数据以及。

二、插入数据 INSERT

1. 插入完整数据(顺序插入)

语法一:

INSERT INTO 表名(字段1,字段2,字段3…字段n) VALUES(值1,值2,值3…值n);

语法二:

INSERT INTO 表名 VALUES (值1,值2,值3…值n);

2. 指定字段插入数据

语法一:

INSERT INTO 表名(字段1,字段2,字段3…) VALUES (值1,值2,值3…);

语法三:推荐使用

INSERT INTO 表名 set 字段1=值1,字段2=值2,字段3=值3...;

比如:

insert into book set name='玄冥神掌',author='玄冥二老'

使用这种方式,可以提高插入速度

3. 插入多条记录

语法:

INSERT INTO 表名 VALUES
(值1,值2,值3…值n),
(值1,值2,值3…值n),
(值1,值2,值3…值n);

4. 插入查询结果

语法:

INSERT INTO 表名(字段1,字段2,字段3…字段n)
SELECT (字段1,字段2,字段3…字段n) FROM 表2
WHERE …;

三、更新数据UPDATE

语法:

UPDATE 表名 SET
字段1=值1,
字段2=值2,
WHERE CONDITION;

示例:

UPDATE mysql.user SET password=password('123')
where user='root' and host='localhost';

  

四、删除数据DELETE

语法:

DELETE FROM 表名
WHERE CONITION;

示例:

DELETE FROM mysql.user
WHERE password='';

delete一般和where配合使用,如果没有where,表示清空表数据。

五、清空数据TRUNCATE TABLE

语法:

TRUNCATE TABLE 表名;

示例:

TRUNCATE TABLE book;

注意:truncate table 在清理大表时,速度非常快!

使用truncate table 清理上千万数据时,存储空间不会立即释放。

这是因为删除操作后在数据文件中留下碎片所致,需要使用命令

OPTIMIZE TABLE 表名;

整理数据文件的碎片,在执行过程中,会锁表!请谨慎操作!

delete * from和truncate table都能清空表数据,那么二者之间的区别在于:

truncate table 不仅是删除表里面的数据,而且还会清空表里面主键的标识。也就是说使用过truncate table 的表在重新写入数据的时候,标识符会从0或1重新开始(看你设置的种子号)。

delete * from就是仅仅能删除数据,不能清空标识。不过delete * from可以后面加Where truncate table却不能加Where。。

三、单表查询

本节重点:

  单表查询

一、单表查询的语法

SELECT 字段1,字段2... FROM 表名
WHERE 条件
GROUP BY field
HAVING 筛选
ORDER BY field
LIMIT 限制条数

二、关键字的执行优先级(重点)

重点中的重点:关键字的执行优先级

从上到下的顺序依次是:
from
where
group by
having
select
distinct
order by
limit

解释:

1.找到表:from
2.拿着where指定的约束条件,去文件/表中取出一条条记录
3.将取出的一条条记录进行分组group by,如果没有group by,则整体作为一组
4.将分组的结果进行having过滤
5.执行select
6.去重
7.将结果按条件排序:order by
8.限制结果的显示条数

  

创建公司员工表,表的字段和数据类型

employee(员工表)

员工id id int
姓名 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

创建员工表,并插入记录

#创建表,设置字段的约束条件
create table employee(
id int primary key 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,#一个部门一个房间
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 |
| emp_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 | |
| salart | double(15,2) | YES | | NULL | |
| office | int(11) | YES | | NULL | |
| depart_id | int(11) | YES | | NULL | |
+--------------+-----------------------+------+-----+---------+----------------+
rows in set (0.08 sec) #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),
('xiaomage','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)
;

(1)where 约束

where子句中可以使用
1.比较运算符:>、<、>=、<=、<>、!=
2.between 80 and 100 :值在80到100之间
3.in(80,90,100)值是80或90或100
4.like 'xiaomagepattern': pattern可以是%或者_。%小时任意多字符,_表示一个字符
5.逻辑运算符:在多个条件直接可以使用逻辑运算符 and or not

验证结果:where约束

1. 单条件查询

mysql> select id,name from employee where id > 5;
+----+------------+
| id | name |
+----+------------+
| 6 | jingliyang |
| 7 | jinxin |
| 8 | xiaomage |
| 9 | 歪歪 |
| 10 | 丫丫 |
| 11 | 丁丁 |
| 12 | 星星 |
| 13 | 格格 |
| 14 | 张野 |
| 15 | 程咬金 |
| 16 | 程咬银 |
| 17 | 程咬铜 |
| 18 | 程咬铁 |
+----+------------+
13 rows in set (0.00 sec)

2. 多条件查询

#查询老师薪资大于1万的名字
mysql> select name from employee where post='teacher' and salary>10000;
+--------+
| name |
+--------+
| alex |
| jinxin |
+--------+
2 rows in set (0.00 sec)

3.关键字BETWEEN AND

举例1:

#查询薪资在1万到2万之间的,结果显示姓名和工资
SELECT name,salary FROM employee
WHERE salary BETWEEN 10000 AND 20000; #结果如下:
+-----------+----------+
| name | salary |
+-----------+----------+
| xiaomage | 10000.00 |
| 张野 | 10000.13 |
| 程咬金 | 20000.00 |
| 程咬银 | 19000.00 |
| 程咬铜 | 18000.00 |
| 程咬铁 | 17000.00 |
+-----------+----------+
6 rows in set (0.00 sec)

举例2:

#查询工资不在1万到2万之间的,结果显示姓名和工资
SELECT name,salary FROM employee
WHERE salary NOT BETWEEN 10000 AND 20000; #结果如下:
+------------+------------+
| name | salary |
+------------+------------+
| egon | 7300.33 |
| alex | 1000000.31 |
| wupeiqi | 8300.00 |
| yuanhao | 3500.00 |
| liwenzhou | 2100.00 |
| jingliyang | 9000.00 |
| jinxin | 30000.00 |
| 歪歪 | 3000.13 |
| 丫丫 | 2000.35 |
| 丁丁 | 1000.37 |
| 星星 | 3000.29 |
| 格格 | 4000.33 |
+------------+------------+
12 rows in set (0.00 sec)

4. 注意''是空字符串,不是null

SELECT name,post_comment FROM employee WHERE post_comment='';

执行结果为空

Empty set (0.00 sec)

执行

update employee set post_comment='' where id=2;

再用上条查看,就会有结果了

mysql> update employee set post_comment='' where id=2;
Query OK, 1 row affected (0.10 sec)
Rows matched: 1 Changed: 1 Warnings: 0 #查看职位描述为空的,显示姓名和职位描述
mysql> SELECT name,post_comment FROM employee WHERE post_comment='';
+------+--------------+
| name | post_comment |
+------+--------------+
| alex | |
+------+--------------+
1 row in set (0.00 sec)

5. 关键字IN集合查询

举例1:

#查询薪资为3千、3千5、4千、9千的,显示姓名和薪资
mysql> SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
+------------+---------+
| name | salary |
+------------+---------+
| yuanhao | 3500.00 |
| jingliyang | 9000.00 |
+------------+---------+
2 rows in set (0.00 sec)

举例2:

#需求同上,推荐使用这种写法
mysql> SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ;
+------------+---------+
| name | salary |
+------------+---------+
| yuanhao | 3500.00 |
| jingliyang | 9000.00 |
+------------+---------+
2 rows in set (0.00 sec)

举例3:

#查询薪资不是3000,3500,4000,9000的,显示姓名和薪资
mysql> SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ;
+-----------+------------+
| name | salary |
+-----------+------------+
| egon | 7300.33 |
| alex | 1000000.31 |
| wupeiqi | 8300.00 |
| liwenzhou | 2100.00 |
| jinxin | 30000.00 |
| xiaomage | 10000.00 |
| 歪歪 | 3000.13 |
| 丫丫 | 2000.35 |
| 丁丁 | 1000.37 |
| 星星 | 3000.29 |
| 格格 | 4000.33 |
| 张野 | 10000.13 |
| 程咬金 | 20000.00 |
| 程咬银 | 19000.00 |
| 程咬铜 | 18000.00 |
| 程咬铁 | 17000.00 |
+-----------+------------+
16 rows in set (0.00 sec)

6. 关键字LIKE模糊查询

通配符'%'

#查询姓名是jin开头的记录
mysql> SELECT * FROM employee WHERE name LIKE 'jin%';
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
2 rows in set (0.00 sec)

通配符'_'

注意:'_'只匹配一个字符

#查询以ale开头,结尾只有一个字符的,结果返回姓名和年龄
mysql> SELECT name,age FROM employee WHERE name LIKE 'ale_';
+------+-----+
| name | age |
+------+-----+
| alex | 78 |
+------+-----+
1 row in set (0.00 sec)

练习:

1. 查看岗位是teacher的员工姓名、年龄
2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
3. 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资
4. 查看岗位描述不为NULL的员工信息
5. 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
6. 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资
7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪

对应的sql语句

select name,age from employee where post = 'teacher';
select name,age from employee where post='teacher' and age > 30;
select name,age,salary from employee where post='teacher' and salary between 9000 and 10000;
select * from employee where post_comment is not null;
select name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);
select name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);
select name,salary*12 from employee where post='teacher' and name like 'jin%';

(2)group by 分组查询

#1、首先明确一点:分组发生在where之后,即分组是基于where之后得到的记录而进行的

#2、分组指的是:将所有记录按照某个相同字段进行归类,比如针对员工信息表的职位分组,或者按照性别进行分组等

#3、为何要分组呢?
取每个部门的最高工资
取每个部门的员工数
取男人数和女人数 小窍门:‘每’这个字后面的字段,就是我们分组的依据 #4、大前提:
可以按照任意字段分组,但是分组完毕后,比如group by post,只能查看post字段,如果想查看组内信息,需要借助于聚合函数

当执行以下sql语句的时候,是以post字段查询了组中的第一条数据,没有任何意义,因为我们现在想查出当前组的多条记录。

mysql> select * from employee group by post;
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 |
| 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
4 rows in set (0.00 sec)

查看MySQL 5.7默认的sql_mode如下:

mysql> select @@global.sql_mode;
+---------------------------------------------------------------+
| @@global.sql_mode |
+---------------------------------------------------------------+
| STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION |
+---------------------------------------------------------------+
1 row in set (0.00 sec)

由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果,默认都是组内的第一条记录,但其实这是没有意义的
如果想分组,则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY

mysql> set global sql_mode='ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION';
Query OK, 0 rows affected, 1 warning (0.00 sec)

设置成功后,一定要退出,然后重新登录方可生效

mysql> exit;
Bye

继续验证通过group by分组之后,只能查看当前字段,如果想查看组内信息,需要借助于聚合函数

#报错
mysql> select * from employee group by post;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'db1.employee.id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by #正确写法
mysql> select post from employee group by post;
+-----------------------------------------+
| post |
+-----------------------------------------+
| operation |
| sale |
| teacher |
| 老男孩驻沙河办事处外交大使 |
+-----------------------------------------+
4 rows in set (0.00 sec)

(3)聚合函数

max()求最大值
min()求最小值
avg()求平均值
sum() 求和
count() 求总个数

强调:聚合函数聚合的是组的内容,若是没有分组,则默认一组

举例:

# 每个部门有多少个员工
mysql> select post,count(id) from employee group by post;
+-----------------------------------------+-----------+
| post | count(id) |
+-----------------------------------------+-----------+
| operation | 5 |
| sale | 5 |
| teacher | 7 |
| 老男孩驻沙河办事处外交大使 | 1 |
+-----------------------------------------+-----------+
4 rows in set (0.00 sec) # 每个部门的最高薪水
mysql> select post,max(salary) from employee group by post;
+-----------------------------------------+-------------+
| post | max(salary) |
+-----------------------------------------+-------------+
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 老男孩驻沙河办事处外交大使 | 7300.33 |
+-----------------------------------------+-------------+
4 rows in set (0.00 sec) # 每个部门的最低薪水
mysql> select post,min(salary) from employee group by post;
+-----------------------------------------+-------------+
| post | min(salary) |
+-----------------------------------------+-------------+
| operation | 10000.13 |
| sale | 1000.37 |
| teacher | 2100.00 |
| 老男孩驻沙河办事处外交大使 | 7300.33 |
+-----------------------------------------+-------------+
4 rows in set (0.00 sec) # 每个部门的平均薪水
mysql> select post,avg(salary) from employee group by post;
+-----------------------------------------+---------------+
| post | avg(salary) |
+-----------------------------------------+---------------+
| operation | 16800.026000 |
| sale | 2600.294000 |
| teacher | 151842.901429 |
| 老男孩驻沙河办事处外交大使 | 7300.330000 |
+-----------------------------------------+---------------+
4 rows in set (0.00 sec) # 每个部门的所有薪水
mysql> select post,sum(age) from employee group by post;
+-----------------------------------------+----------+
| post | sum(age) |
+-----------------------------------------+----------+
| operation | 100 |
| sale | 150 |
| teacher | 344 |
| 老男孩驻沙河办事处外交大使 | 18 |
+-----------------------------------------+----------+
4 rows in set (0.00 sec)

 注意:聚合函数,可以用在任何地方

给使用聚合函数的字段起别名

select post,avg(salary) as average from employee group by post having average > 15000;

小练习

1. 查询岗位名以及岗位包含的所有员工名字
2. 查询岗位名以及各岗位内包含的员工个数
3. 查询公司内男员工和女员工的个数
4. 查询岗位名以及各岗位的平均薪资
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
7. 查询男员工与男员工的平均薪资,女员工与女员工的平均薪资

答案:

#题1:分组
mysql> select post,group_concat(name) from employee group by post;
+-----------------------------------------+---------------------------------------------------------+
| post | group_concat(name) |
+-----------------------------------------+---------------------------------------------------------+
| operation | 张野,程咬金,程咬银,程咬铜,程咬铁 |
| sale | 歪歪,丫丫,丁丁,星星,格格 |
| teacher | alex,wupeiqi,yuanhao,liwenzhou,jingliyang,jinxin,成龙 |
| 老男孩驻沙河办事处外交大使 | egon |
+-----------------------------------------+---------------------------------------------------------+ #题目2:
mysql> select post,count(id) from employee group by post;
+-----------------------------------------+-----------+
| post | count(id) |
+-----------------------------------------+-----------+
| operation | 5 |
| sale | 5 |
| teacher | 7 |
| 老男孩驻沙河办事处外交大使 | 1 |
+-----------------------------------------+-----------+ #题目3:
mysql> select sex,count(id) from employee group by sex;
+--------+-----------+
| sex | count(id) |
+--------+-----------+
| male | 10 |
| female | 8 |
+--------+-----------+ #题目4:
mysql> select post,avg(salary) from employee group by post;
+-----------------------------------------+---------------+
| post | avg(salary) |
+-----------------------------------------+---------------+
| operation | 16800.026000 |
| sale | 2600.294000 |
| teacher | 151842.901429 |
| 老男孩驻沙河办事处外交大使 | 7300.330000 |
+-----------------------------------------+---------------+ #题目5
mysql> select post,max(salary) from employee group by post;
+-----------------------------------------+-------------+
| post | max(salary) |
+-----------------------------------------+-------------+
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 老男孩驻沙河办事处外交大使 | 7300.33 |
+-----------------------------------------+-------------+ #题目6
mysql> select post,min(salary) from employee group by post;
+-----------------------------------------+-------------+
| post | min(salary) |
+-----------------------------------------+-------------+
| operation | 10000.13 |
| sale | 1000.37 |
| teacher | 2100.00 |
| 老男孩驻沙河办事处外交大使 | 7300.33 |
+-----------------------------------------+-------------+ #题目七
mysql> select sex,avg(salary) from employee group by sex;
+--------+---------------+
| sex | avg(salary) |
+--------+---------------+
| male | 110920.077000 |
| female | 7250.183750 |
+--------+---------------+

(4)HAVING过滤

HAVING与WHERE不一样的地方在于

#!!!执行优先级从高到低:where > group by > having
#1. Where 发生在分组group by之前,因而Where中可以有任意字段,但是绝对不能使用聚合函数。 #2. Having发生在分组group by之后,因而Having中可以使用分组的字段,无法直接取到其他字段,可以使用聚合函数

验证:

#查询薪资大于100万的
mysql> select * from employee where salary>1000000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
1 row in set (0.00 sec) #查询薪资大于100万的,效果同上
mysql> select * from employee having salary>1000000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
1 row in set (0.00 sec) #错误,分组后无法直接取到salary字段
#必须使用group by才能使用group_concat()函数,将所有的name值连接
mysql> select post,group_concat(name) from employee group by post having salary > 10000;
ERROR 1054 (42S22): Unknown column 'salary' in 'having clause' #正确写法
mysql> select post,group_concat(name),salary from employee group by post,name,salary having salary > 10000;
+-----------+--------------------+------------+
| post | group_concat(name) | salary |
+-----------+--------------------+------------+
| operation | 张野 | 10000.13 |
| operation | 程咬金 | 20000.00 |
| operation | 程咬铁 | 17000.00 |
| operation | 程咬铜 | 18000.00 |
| operation | 程咬银 | 19000.00 |
| teacher | alex | 1000000.31 |
| teacher | jinxin | 30000.00 |
+-----------+--------------------+------------+
7 rows in set (0.00 sec)

小练习:

1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
2. 查询各岗位平均薪资大于10000的岗位名、平均工资
3. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资

小练习答案:

# 题1:
mysql> select post,group_concat(name),count(id) from employee group by post;
+-----------------------------------------+-----------------------------------------------------------+-----------+
| post | group_concat(name) | count(id) |
+-----------------------------------------+-----------------------------------------------------------+-----------+
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 | 5 |
| sale | 格格,星星,丁丁,丫丫,歪歪 | 5 |
| teacher | xiaomage,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex | 7 |
| 老男孩驻沙河办事处外交大使 | egon | 1 |
+-----------------------------------------+-----------------------------------------------------------+-----------+
rows in set (0.00 sec) mysql> select post,group_concat(name),count(id) from employee group by post having count(id)<;
+-----------------------------------------+--------------------+-----------+
| post | group_concat(name) | count(id) |
+-----------------------------------------+--------------------+-----------+
| 老男孩驻沙河办事处外交大使 | egon | 1 |
+-----------------------------------------+--------------------+-----------+
row in set (0.00 sec) #题2:
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;
+-----------+---------------+
| post | avg(salary) |
+-----------+---------------+
| operation | 16800.026000 |
| teacher | 151842.901429 |
+-----------+---------------+
rows in set (0.00 sec) #题3:
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <;
+-----------+--------------+
| post | avg(salary) |
+-----------+--------------+
| operation | 16800.026000 |
+-----------+--------------+
row in set (0.00 sec)

(5)order by 查询排序

按单列排序
SELECT * FROM employee ORDER BY age;
SELECT * FROM employee ORDER BY age ASC;
SELECT * FROM employee ORDER BY age DESC;
按多列排序:先按照age升序排序,如果年纪相同,则按照id降序
SELECT * from employee
ORDER BY age ASC,
id DESC;

验证多列排序

#先根据age进行升序排列,再根据id进行降序排列
SELECT * from employee ORDER BY age ASC,id DESC;
mysql> SELECT * from employee ORDER BY age ASC,id DESC;
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 |
| 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 |
| 16 | 程咬银 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 |
| 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
| 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 |
| 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 |
| 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
| 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 |
| 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 |
| 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 |
| 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 8 | xiaomage | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 |
| 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 |
| 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
rows in set (0.01 sec)

小练习:

1. 查询所有员工信息,先按照age升序排序,如果age相同则按照hire_date降序排序
2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列 

小练习答案

# 题目1
select * from employee ORDER BY age asc,hire_date desc; # 题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;
+-----------+---------------+
| post | avg(salary) |
+-----------+---------------+
| operation | 16800.026000 |
| teacher | 151842.901429 |
+-----------+---------------+
rows in set (0.00 sec) # 题目3
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;
+-----------+---------------+
| post | avg(salary) |
+-----------+---------------+
| teacher | 151842.901429 |
| operation | 16800.026000 |
+-----------+---------------+
rows in set (0.00 sec)

(5)limit  限制查询的记录数:

示例:

##默认初始位置为0
SELECT * FROM employee ORDER BY salary DESC
LIMIT 3; #从第0开始,即先查询出第一条,然后包含这一条在内往后查5条
SELECT * FROM employee ORDER BY salary DESC
LIMIT 0,5; #从第5开始,即先查询出第6条,然后包含这一条在内往后查5条
SELECT * FROM employee ORDER BY salary DESC
LIMIT 5,5;

小练习:

分页显示,每页5条

小练习答案

# 第1页数据
mysql> select * from employee limit 0,5;
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 |
| 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 |
| 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
rows in set (0.00 sec)
# 第2页数据
mysql> select * from employee limit 5,5;
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
| 8 | xiaomage | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 |
| 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
rows in set (0.00 sec)
# 第3页数据
mysql> select * from employee limit 10,5;
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 |
| 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 |
| 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 |
| 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
rows in set (0.00 sec)

mysql分页计算总页数公式

总记录数:totalRecord
每页最大记录数:maxResult
totalPage = (totalRecord + maxResult -1) / maxResult; 其中 maxResult - 1 就是 totalRecord / maxResult 的最大的余数

四、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

多表查询,就是为了解决外键问题

建表的时候,不需要考虑表的创建顺序,因为没有约束关系!

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

#删除已存在的表
mysql> drop table department;
Query OK, 0 rows affected (0.19 sec) #删除已存在的表
mysql> drop table employee;
Query OK, 0 rows affected (0.19 sec) #创建部门表
create table department(
id int,
name varchar(20)
); #创建员工表
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)
; #查看部门表结构
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
rows in set (0.00 sec) #查看员工表结构
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 | |
+--------+-----------------------+------+-----+---------+----------------+
rows in set (0.00 sec) #查看部门表记录
mysql> select * from department;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+
rows in set (0.00 sec) #查看员工表记录
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 |
+----+----------+--------+------+--------+
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,多表查询一定要加表名

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,请参考以下链接

https://baike.baidu.com/item/%E7%AC%9B%E5%8D%A1%E5%B0%94%E4%B9%98%E7%A7%AF/6323173?fr=aladdin&fromid=1434391&fromtitle=%E7%AC%9B%E5%8D%A1%E5%B0%94%E7%A7%AF

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 |
+----+----------+--------+------+--------+------+--------------+
24 rows in set (0.00 sec)

(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 |
+----+---------+------+--------+--------------+
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 |
+----+----------+--------------+
rows in set (0.00 sec)

从结果上来看,比较符合现实情况。left后面还可以再接一个表,比如三表查询

#注意: test表,此时不存在!
select * from employee left join department on employee.dep_id = department.id;
select * from department left join employee on employee.dep_id = department.id
left join test on test.id=department.id;

(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 |
+------+---------+--------------+
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 |
+------+----------+--------+------+--------+------+--------------+
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,employee.age,department.name from employee
inner join department on employee.dep_id = department.id
where employee.age > 25; #返回结果
+---------+------+--------------+
| name | age | name |
+---------+------+--------------+
| alex | 48 | 人力资源 |
| wupeiqi | 38 | 人力资源 |
| yuanhao | 28 | 销售 |
+---------+------+--------------+
3 rows in set (0.00 sec)

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; #返回结果
+----+---------+------+--------------+
| id | name | age | name |
+----+---------+------+--------------+
| 4 | yuanhao | 28 | 销售 |
| 3 | wupeiqi | 38 | 人力资源 |
| 2 | alex | 48 | 人力资源 |
+----+---------+------+--------------+
3 rows in set (0.00 sec)

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