本章节将总结MySQL的表结构修改，约束条件，表关系。
表结构修改：
首先查看一下表结构：
desc tb_name; 查询表结构
mysql> desc student;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
修改表结构，就是对表结构进行增删改查。关键字为alter
1、修改表结构：alter table tb_name rename new_tb_name;
2、修改字段名：alter table tb_name change name new_name data_type;
3、修改字段类型：alter table tb_name modify field_name data_type;
4、添加字段：alter table tb_name add [col] field_name data_type;
5、删除字段：alter table tb_name drop [col] field_name;
其中，tb_name为要修改的表名，new_tb_name为修改后的新表名，data_type为字段的类型，new_name 为新的字段名，field_name为字段名。
mysql> show tables;
+------------------------+
| Tables_in_python_study |
+------------------------+
| class |
| student |
+------------------------+
2 rows in set (0.00 sec)
mysql> alter table student rename stu; #修改表名
Query OK, 0 rows affected (0.14 sec)
mysql> show tables;
+------------------------+
| Tables_in_python_study |
+------------------------+
| class |
| stu |
+------------------------+
2 rows in set (0.00 sec)
mysql> desc stu;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
mysql> alter table stu change id s_id tinyint; #修改字段名以及类型。
Query OK, 13 rows affected (1.05 sec)
Records: 13 Duplicates: 0 Warnings: 0
mysql> desc stu;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| s_id | tinyint(4) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> desc stu;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| s_id | tinyint(4) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> alter table stu modify name char(20); #修改字段类型
Query OK, 13 rows affected (0.90 sec)
Records: 13 Duplicates: 0 Warnings: 0
mysql> desc stu;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| s_id | tinyint(4) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> desc stu;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| s_id | tinyint(4) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> alter table stu add phone bigint; #增加字段
Query OK, 0 rows affected (0.65 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc stu;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| s_id | tinyint(4) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
| phone | bigint(20) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
mysql> desc stu;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| s_id | tinyint(4) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
| phone | bigint(20) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
mysql> alter table stu drop phone; #删除字段
Query OK, 0 rows affected (0.67 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc stu;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| s_id | tinyint(4) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| grade | varchar(6) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
约束条件：
约束是一种限制，通过对表中的数据做出限制，来确保表中的数据完整性，唯一性。
| 约束类型 | 关键字 |
|---|---|
| 默认 | default |
| 非空 | not null |
| 唯一 | unique key |
| 自增长 | auto_increment 和主键联合使用 |
| 主键 | primary key |
| 外键 | foreign key |
外键的约束：
B表中的id字段，只能添加A表中id有的字段值。如果没有，将无法添加。
A表中的id字段是被参照的数据，不能被修改和删除。
我们以A表为student表，B为class表为例。
mysql> create table student(
-> id int primary key auto_increment,
-> name varchar(20) not null,
-> age tinyint,
-> sex enum('M','F') default 'M',
-> grade varchar(6),
-> id_card bigint unique key
-> );
Query OK, 0 rows affected (0.39 sec)
mysql> create table class(
-> c_id int primary key auto_increment,
-> c_name varchar(20) not null,
-> foreign key(c_id) references student(id)
-> );
Query OK, 0 rows affected (0.33 sec)
mysql> insert into student(name,grade,id_card)values
-> ('apple','1','123456789'),
-> ('pear','2','213456789');
Query OK, 2 rows affected (0.06 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from student;
+----+-------+------+------+-------+-----------+
| id | name | age | sex | grade | id_card |
+----+-------+------+------+-------+-----------+
| 1 | apple | NULL | M | 1 | 123456789 |
| 2 | pear | NULL | M | 2 | 213456789 |
+----+-------+------+------+-------+-----------+
2 rows in set (0.00 sec)
mysql> insert into class values('class1'),('class2');
ERROR 1136 (21S01): Column count doesn't match value count at row 1
mysql> insert into class(c_name) values('class1'),('class2');
Query OK, 2 rows affected (0.04 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from class;
+------+--------+
| c_id | c_name |
+------+--------+
| 1 | class1 |
| 2 | class2 |
+------+--------+
2 rows in set (0.01 sec)
mysql> alter table student drop id;
ERROR 1829 (HY000): Cannot drop column 'id': needed in a foreign key constraint 'class_ibfk_1' of table 'db_name.class'
表关系
表关系可以分为：
一对一、一对多、多对多。
图1
举例，学生表中有学号、姓名、学院，但学生还有些比如电话，家庭住址等比较私密的信息，这些信息不会放在学生表当中，会新建一个学生的详细信息表来存放。
这时的学生表和学生的详细信息表两者的关系就是一对一的关系，因为一个学生只有一条详细信息。用主键加主键的方式来实现这种关系。
1、一对一：用外键的方式，把两个表的主键关联。
2、一对多：通过外键来实现表的一对多关系。
3、多对多：多对多的关系需要创建中间表来实现。
mysql> desc student;
+---------+---------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | enum('M','F') | YES | | M | |
| grade | varchar(6) | YES | | NULL | |
| id_card | bigint(20) | YES | UNI | NULL | |
+---------+---------------+------+-----+---------+----------------+
6 rows in set (0.01 sec)
mysql> create table course(
-> cou_id int primary key auto_increment,
-> cou_name varchar(20) not null,
-> cou_teacher varchar(20) not null,
-> foreign key(cou_id) references student(id)
-> );
Query OK, 0 rows affected (0.31 sec)
mysql> insert into course (cou_name,cou_teacher)values('python','tony'),('java','matin');
Query OK, 2 rows affected (0.05 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from course;
+--------+----------+-------------+
| cou_id | cou_name | cou_teacher |
+--------+----------+-------------+
| 1 | python | tony |
| 2 | java | matin |
+--------+----------+-------------+
2 rows in set (0.01 sec)
mysql> create table curr( #创建中间表，实现多对多的关系
-> id int,
-> cou_id int ,
-> primary key(id,cou_id),
-> foreign key(id) references student(id),
-> foreign key(cou_id) references course(cou_id)
-> );
Query OK, 0 rows affected (0.35 sec)
mysql> insert into curr values(1,2),(2,1);
Query OK, 2 rows affected (0.03 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from curr;
+----+--------+
| id | cou_id |
+----+--------+
| 2 | 1 |
| 1 | 2 |
+----+--------+
2 rows in set (0.00 sec)
mysql> select s.name,c.cou_name,c.cou_teacher from student s,course c ,curr cu where s.id=cu.id and c.cou_id=cu.cou_id;
+-------+----------+-------------+
| name | cou_name | cou_teacher |
+-------+----------+-------------+
| pear | python | tony |
| apple | java | matin |
+-------+----------+-------------+
2 rows in set (0.00 sec)