mysql foreignkey

1.foreign key

当数据足够大的时候,字段会出现大量重复,

解决:额外定义一个大量冗余的字段表,(有id) 一张是关联表(从表),一张是被关联表(主表)

进行关联的时候 ,先创建被关联表, 现在被关联表添加,再是关联表

constraint fk_dep(随便写) foreign key(dep_id) references dep(id)

在创建关联表 但是再修改出现问题是先删除这个部门的员工才能删除这个部门
问题代码:

 #1.创建表时先创建被关联表，再创建关联表
 # 先创建被关联表（dep表）
 create table dep(
     id int primary key,
     name varchar(20) not null,
     descripe varchar(20) not null
 );

 #再创建关联表（emp表）
 create table emp(
     id int primary key,
     name varchar(20) not null,
     age int not null,
     dep_id int,
     constraint fk_dep foreign key(dep_id) references dep(id)
 );

 #2.插入记录时，先往被关联表中插入记录，再往关联表中插入记录

 insert into dep values
 (1,'IT','IT技术有限部门'),
 (2,'销售部','销售部门'),
 (3,'财务部','花钱太多部门');

 insert into emp values
 (1,'zhangsan',18,1),
 (2,'lisi',19,1),
 (3,'egon',20,2),
 (4,'yuanhao',40,3),
 (5,'alex',18,2);

 3.删除表
 #按道理来说，删除了部门表中的某个部门，员工表的有关联的记录相继删除。
 mysql> delete from dep where id=3;
 ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`db5`.`emp`, CONSTRAINT `fk_name` FOREIGN KEY (`dep_id`) REFERENCES `dep` (`id`))

 #但是先删除员工表的记录之后，再删除当前部门就没有任何问题

 mysql> delete from emp where dep =3;
 Query OK, 1 row affected (0.00 sec)

 mysql> select * from emp;
 +----+----------+-----+--------+
 | id | name     | age | dep_id |
 +----+----------+-----+--------+
 |  1 | zhangsan |  18 |      1 |
 |  2 | lisi     |  18 |      1 |
 |  3 | egon     |  20 |      2 |
 |  5 | alex     |  18 |      2 |
 +----+----------+-----+--------+
 rows in set (0.00 sec)

 mysql> delete from dep where id=3;
 Query OK, 1 row affected (0.00 sec)

 mysql> select * from dep;
 +----+-----------+----------------------+
 | id | name      | descripe             |
 +----+-----------+----------------------+
 |  1 | IT        | IT技术有限部门       |
 |  2 | 销售部    | 销售部门             |
 +----+-----------+----------------------+
 rows in set (0.00 sec)

删除员工才能删除部门

解决办法:
关联表升级:

    on delete cascade #同步删除
    on update cascade #同步更新
就可以同步删除了

 create table emp(
     id int primary key,
     name varchar(20) not null,
     age int not null,
     dep_id int,
     constraint fk_dep foreign key(dep_id) references dep(id)
     on delete cascade #同步删除
     on update cascade #同步更新
 );

修改的emp

后面的同步删除,更新

 #再去删被关联表(dep)的记录，关联表(emp)中的记录也跟着删除
 mysql> delete from dep where id=3;
 Query OK, 1 row affected (0.00 sec)

 mysql> select * from dep;
 +----+-----------+----------------------+
 | id | name      | descripe             |
 +----+-----------+----------------------+
 |  1 | IT        | IT技术有限部门       |
 |  2 | 销售部    | 销售部门             |
 +----+-----------+----------------------+
 rows in set (0.00 sec)

 mysql> select * from emp;
 +----+----------+-----+--------+
 | id | name     | age | dep_id |
 +----+----------+-----+--------+
 |  1 | zhangsan |  18 |      1 |
 |  2 | lisi     |  19 |      1 |
 |  3 | egon     |  20 |      2 |
 |  5 | alex     |  18 |      2 |
 +----+----------+-----+--------+
 rows in set (0.00 sec)

 #再去更改被关联表(dep)的记录，关联表(emp)中的记录也跟着更改

 mysql> update dep set id=222 where id=2;
 Query OK, 1 row affected (0.02 sec)
 Rows matched: 1  Changed: 1  Warnings: 0

 # 赶紧去查看一下两张表是否都被删除了，是否都被更改了
 mysql> select * from dep;
 +-----+-----------+----------------------+
 | id  | name      | descripe             |
 +-----+-----------+----------------------+
 |   1 | IT        | IT技术有限部门       |
 | 222 | 销售部    | 销售部门             |
 +-----+-----------+----------------------+
 rows in set (0.00 sec)

 mysql> select * from emp;
 +----+----------+-----+--------+
 | id | name     | age | dep_id |
 +----+----------+-----+--------+
 |  1 | zhangsan |  18 |      1 |
 |  2 | lisi     |  19 |      1 |
 |  3 | egon     |  20 |    222 |
 |  5 | alex     |  18 |    222 |
 +----+----------+-----+--------+
 rows in set (0.00 sec)

同步删除,更新

2.

外键的变种 三种关系

因为有foreign key 的约束 ,使表有了三种关系

多对一  书 出版商 (一个出版商可以出版多种书  而一本书只能由一个出版商出,因为版权)

先创建出版商,在创建书 (含有

 constraint fk_book_press foreign key(press_id) references press(id)
    on delete cascade
    on update cascade
);

)

 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）

多对一

多对多 书 作者  (作者可以写多本书,书也可由多个作者)

这样是无法创建foreign ke关系的,可以创建第三张表存放

1.创建书 和 作者 2.创建出一个表明关系的表  3.放入作者和作者的作品.4.在公共表放上数据(

insert into author2book(author_id,book_id) values

)

 # 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）

多对多

一对一

在其中从表的仿造主表加上unique

 #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
 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;

 用户和博客（一对一）

一对一

二.

单表查询

语法 select 字段1,字段2 ...(*) from 表名

where 条件

group by

（1）where 约束　

where子句中可以使用

1.比较运算符>,<,>=,<=,<>,!=

2.between 80  and 100;(前面是)需要查找的条件

3. in (80,90,100);值再()中间

4.like"jin%" %表示查找多个含有jin的字符

like"ale_" _表示查找一个含有ale关键字的字符

5.逻辑运算符: 可以再多个条件下使用逻辑运算符 and or  not

 #1 :单条件查询
 mysql> select id,emp_name from employee where id > 5;
 +----+------------+
 | id | emp_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     |

 #2 多条件查询
 mysql> select emp_name from employee where post='teacher' and salary>10000;
 +----------+
 | emp_name |
 +----------+
 | alex         |
 | jinxin     |
 +----------+

 #3.关键字BETWEEN AND
  SELECT name,salary FROM employee
         WHERE salary BETWEEN 10000 AND 20000;

  SELECT name,salary FROM employee
         WHERE salary NOT BETWEEN 10000 AND 20000;

 #注意''是空字符串，不是null
  SELECT name,post_comment FROM employee WHERE post_comment='';
  ps：
         执行
         update employee set post_comment='' where id=2;
         再用上条查看，就会有结果了
 #5:关键字IN集合查询
 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 |
 +------------+---------+
 rows in set (0.00 sec)

 mysql>  SELECT name,salary FROM employee  WHERE salary IN (3000,3500,4000,9000) ;
 +------------+---------+
 | name       | salary  |
 +------------+---------+
 | yuanhao    | 3500.00 |
 | jingliyang | 9000.00 |
 +------------+---------+
 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 |
 +-----------+------------+
 rows in set (0.00 sec)

 #6:关键字LIKE模糊查询
 通配符’%’
 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 |
 +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
 rows in set (0.00 sec)

 通配符'_'

 mysql> SELECT  age FROM employee WHERE name LIKE 'ale_';
 +-----+
 | age |
 +-----+
 |  78 |
 +-----+
 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%';

 where约束

where 详解以及练习

（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

在分组前需要进行

mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
也可以将

ONLY_FULL_GROUP_BY 放置配置文件上

聚合函数辅助分组

 mysql> select * from emp group by post;# 报错
 ERROR 1054 (42S22): Unknown column 'post' in 'group statement'

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

分组