Python学习day43-数据库(多表关系)

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Python学习day43-数据库(多表关系)字段的详细操作外键多表关系一对一一对多多对多

Python学习day43-数据库(多表关系)

字段的详细操作

前面我们学过添加字段以及修改的方式,今天我们主要需要了解的东西是通过alter来实现的.

 

 

 

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create table tf1(

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id int primary key auto_increment,

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x int,

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    y int

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);

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# ALTER TABLE 表名 MODIFY 字段名1 数据类型 FIRST|AFTER 字段名2 

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# 修改,modify可以修改一个已有字段的类型,位置以及默认值,change则是可以直接修改一个已有字段为一个全新的字段

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alter table tf1 modify x char(4) default '';

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alter table tf1 modify x char(4) first;# 把字段x这一列移到最开始

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alter table tf1 modify x char(4) after y int;# 把字段x这一列移到y后面

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alter table tf1 change y m char(4) default '';

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​

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# 增加

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alter table 表名 add 新字段 类型[(长度) 约束条件];# 默认末尾增加

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alter table tf1 add z int unsigned;

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​

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# 头部添加

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alter table 表名 add 新字段 类型[(长度) 约束条件] first;# 头部添加

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​

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# 某字段后添加

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alter table 表名 add 新字段 类型[(长度) 约束条件] after 旧字段名;

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​

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# 删除

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alter table 表名 drop 字段名;

 

 

外键

上节中我们了解到,外键和主键一样,是创建表的时候一种约束条件.其主要作用也和主键比较类似,是索引的一种.但是其和主键不同的一点在于,MySQL会为表中所有的主键进行索引,但是外键字段不会自动索引,只有用户去明确定义这个索引,我们才可以使用.

 

 

 

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# 外键的语句如下

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foreign key(所在表的外键字段) references 关联表(关联字段)

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# 举个栗子:

4

foreign key(detail_id) references author_detail(id)

5

'''

6

注意:

7

1. 外键的字段名是自定义的,但为了索引方便通常将外键名字定义为 关联表_关联字段

8

2. 外键字段本身可以唯一或者不唯一,但是外键所关联的字段一定是唯一的,这点非常重要

9

'''

 

 

多表关系

多表关系,其实多个表之间的关系只有三种,即一对一,一对多和多对多,

举个栗子:

 

 

 

x

 

 

 

 

1

1. 一对一:外键在任何一方定义都可以,此时外键要设置唯一键,即unique

2

作者(author):id,name,gender,age,mobile,info,address

3

作者详情(author_detail):id,info,address,author_id

4

2. 一对多:外键必须放在多的那边,且外键值不唯一

5

书(book):id,name,price,publish_id

6

出版社(publish):id,name,address,phone

7

3. 多对多:一定要创建第三张关系表,每一个外键不唯一,但是多个外键可以建立联合唯一

8

作者(author):id,name,age

9

出版社(publish):id,name,address

10

关系表(author_publish):id,author_id,publish_id

 

 

接下来我们就根据上面的栗子来创建下面的列表.

一对一

 

 

 

x

 

 

 

 

1

'''

2

外键在任何一方定义都可以,此时外键要设置唯一键,即unique

3

'''

4

# 作者详情表

5

create table author_detail(

6

id int primary key auto_increment,

7

    info varchar(256),

8

    address varchar(256)

9

);

10

​

11

# 作者表

12

create table author(

13

id int primary key auto_increment,

14

    name varchar(64) not NUll,

15

    mobile char(11) unique not NUll,

16

    sex enum('male','female')default 'male',

17

    age int default 0,

18

    detail_id int unique not null,# 建立外键字段detail_id,且有唯一值,即不能重复,但其实这只是个普通字段而已,如果没有下面的绑定语句的话.

19

    foreign key(detail_id) references author_detail(id) 

20

    on update cascade

21

    on delete cascade,# 创建外键并建立级联关系

22

);

23

​

24

​

25

# 需要先插入被关联表,然后再插入关联表,否则会插入数据失败

26

insert into author_detail(info,address) values('Tom_info','Tom_address');

27

insert into author(name,mobile,detail_id) values('Tom','123456789',1);

28

​

29

# 修改关联表

30

update author set detail_id=3 where detail_id=2;# 3不存在

31

update author set detail_id=1 where detail_id=2;# 1已经被关联

32

insert into author_detail(info,address) values('Tom_info_sup','Tom_address_sup');

33

update author set detail_id=2 where detail_id=2;

34

​

35

# 删除关联表

36

delete from author detail_id=3;# 直接删除

37

​

38

# 修改被关联表author_detail

39

update author_detail set id=10 where id=1;# 因为有级联关系,所以可以修改,同时关联表里面的detail_id也会修改,实际开发中几乎没有应用场景,因为这只是单纯修改了关联字段的值,关联的本身关系并没有改变,所以几乎没有意义

40

​

41

# 删除被关联表

42

delete from author where detail_id=10;# 可以删除,对被关联表没有影响

43

delete from author_detail where id=10;# 不止author_detail里面的数据删除了,把关联表author里面的记录也级联删除了

 

 

一对多

 

 

 

xxxxxxxxxx

1

51

 

 

 

 

1

'''

2

一对多:外键必须放在多的那边,且外键值不唯一

3

​

4

出版社(publish):id,name,address,phone

5

​

6

书(book):id,name,price,publish_id

7

'''

8

​

9

create table publish(

10

id int primary key auto_increment,

11

    name varchar(64),

12

    address varchar(256),

13

    phone char(20)

14

);

15

​

16

# 书(book):id,name,price,publish_id,author_id

17

create table book(

18

id int primary key auto_increment,

19

    name varchar(64) not null,

20

    price decimal(5,2) default 0,

21

    publish_id int,#一对多的外键不能设置唯一,因为同时有多本书对应出版社,一旦外键设置唯一值,出版社一个id只能出一本书,这是不符合实际情况的

22

    foreign key(publish_id) references publish(id)

23

    on update cascade

24

    on delete cascade

25

);

26

​

27

# 增:先插入被关联表(publish)的数据,然后插入关联表(book)的数据

28

insert into  publish(name,address,phone) values

29

('人民出版社','北京','010-110'),

30

('西交大出版社','西安','010-119'),

31

    ('老男孩出版社','上海','010-120');

32

​

33

insert into  book(name,price,publish_id) values

34

('西游记',6.66,1),

35

('东游记',8.66,1),

36

('Python入门',9.66,2);

37

insert into  book(name,price,publish_id) values

38

('西游记',6.66,4);# 报错,因为publish_id为4不存在

39

​

40

# 更新:直接更新被关联表(publish),关联表(book)外键会 级联更新

41

update publish set id=10 where id=1;

42

# 直接更新关联表(book)的外键,修改的值对应被关联表(publish)的主键如果存在,可以更新成功,否则会失败

43

update book set publish_id=2 where id=4;# 成功

44

update book set publish_id=1 where id=4;# 失败

45

​

46

# 删:

47

# 删被关联表,关联表会被级联删除

48

​

49

delete from publish where id=2;

50

# 删关联表,被关联表不会发生变化

51

delete from book where publish_id=3;

 

 

多对多

 

 

 

xxxxxxxxxx

1

66

 

 

 

 

1

'''

2

多对多:一定要创建第三张关系表,每一个外键不唯一,但是多个外键可以建立联合唯一

3

作者(author):id,name,age

4

出版社(publish):id,name,address

5

作者与出版社关系表:id,author_id,publish_id

6

​

7

关系表关联着作者和出版社两张表,在表结构上,作者与出版社量表键没有任何关系

8

​

9

​

10

'''

11

create table author(

12

id int primary key auto_increment,

13

    name varchar(16),

14

    age int unsigned default 0

15

);

16

​

17

create publish(

18

id int primary key auto_increment,

19

    name varchar(64),

20

    address varcher(256)

21

);

22

​

23

create author_publish(

24

id int primary key auto_increment,

25

    # 关系表一定有多个外键,关联着多张表,还可以有着一些自定义的字段,比如作者和出版社的签约时间,这些字段通常与两者都有关系,且唯一

26

    # 关联作者表

27

    author_id int,

28

    foreign key(author_id) refenrences author(id)

29

    on undate cascade

30

    on delete cascade,

31

    # 关联出版社表

32

    publish_id int,

33

    foreign key(publish_id) refenrences publish(id)

34

    on undate cascade

35

    on delete cascade,

36

    # 建立两个字段的联合唯一,即作者和出版社的绑定关系,同一时间一个作者只能绑定一个出版社,重复绑定会无效

37

    unique(author_id,publish_id)

38

);

39

​

40

​

41

# 增:两张被关联表,没有前后关系,但关系表必须在两个表都提供数据后才能进行关系匹配

42

insert into author(name,age) values

43

('tom',67),

44

('marry',76),

45

('ffx',3);

46

insert into publish(name,address) values

47

('老孩出版社','上海'),

48

('小女孩出版社','北京');

49

50

# 操作关系表:

51

insert into author_publish(author_id,publish_id) values

52

(1,1),

53

(1,2),

54

(2,1),

55

(2,2),

56

(3,1);

57

​

58

# 关系表操作:增,删,改,只要两张被关系表有提供对应的操作数据,都可以操作成功,且对两张被关系的表没有影响

59

​

60

# 操作两张被关系表:

61

#增:两个表独立,不会互相影响

62

insert into publish(name,address) values('西交大出版社','西安');

63

# 改:关系表都会级联更新

64

update publish set id=10 where id=1;

65

# 删:关系表都会级联删除

66

delete from author where name='tom';