（五）mysql表操作和约束条件

（1）表基本操作

1)创建表

create table 表名(字段 数据类型 约束条件)
mysql> create table student1(id int,name varchar(50),sex enum('w','f'),age int);

2)查看表

mysql> desc student1;  #查看表结构
+-------+---------------+------+-----+---------+-------+
| Field | Type          | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| id    | int(11)       | YES  |     | NULL    |       |
| name  | varchar(50)   | YES  |     | NULL    |       |
| sex   | enum('w','f') | YES  |     | NULL    |       |
| age   | int(11)       | YES  |     | NULL    |       |
+-------+---------------+------+-----+---------+-------+

mysql> show create table student1;  #查看创表语句
----------------+| student1 | CREATE TABLE `student1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(50) DEFAULT NULL,
  `sex` enum('w','f') DEFAULT NULL,
  `age` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 

3)删除表

drop table 表名

4)插入数据没有约束条件：数据可以重复

mysql> insert into student1 values(3,'wack','f',21),(4,'jaxc','w',18);
mysql> insert into student1 values(3,'wack','f',21),(4,'jaxc','w',18);
mysql> insert into student1(name,sex) values('wf','w');
mysql> insert into student1(name,sex) values('wf','w');
mysql> select * from student1;
|    4 | jaxc | w    |   18 |
|    3 | wack | f    |   21 |
|    4 | jaxc | w    |   18 |
| NULL | wf   | w    | NULL |
| NULL | wf   | w    | NULL |
+------+------+------+------+

（2）表约束条件：保证数据的完整性和一致性

1)约束条件

primary key ：标识字段为主键，可以唯一的标识记录，特性：唯一，不可以为空，一个表中只有1个主键
foreign key	：标识字段为外键，实现表与表之间的关联
not null：标识该字段不能为空
unique key ：标识该字段唯一，可以为空，一个表中可以有多个unique key
auto_increment：标识该字段的值自动增长（整数类型，而且是主键）
default	：为该字段设置默认值
unsigned ：无符号，正数
zerofill : 使用0填充

2)default默认值和not null非空

mysql> create table student1(id int not null,name varchar(50) not null,sex enum('m','f') default 'm' not null,age int unsigned default 18 not null);
mysql> desc student1;
+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| id    | int(11)          | NO   |     | NULL    |       |
| name  | varchar(50)      | NO   |     | NULL    |       |
| sex   | enum('m','f')    | NO   |     | m       |       |
| age   | int(10) unsigned | NO   |     | 18      |       |
+-------+------------------+------+-----+---------+-------+
mysql> insert into student1 values(1,'jack','m',21);
mysql> insert into student1(id,name) values(2,'robin');
mysql> insert into student1 values(3,null,'f',28);
ERROR 1048 (23000): Column 'name' cannot be null

3)primary key单列做主键测试

创建主键方式1：
mysql> create table student(id int  auto_increment primary key,
    -> name varchar(50) not null,
    -> sex enum('m','f') not null default 'm',
    -> age int not null default 18);

创建主键方式二：
mysql> create table student(id int  auto_increment,
    -> name varchar(50) not null,
    -> sex enum('m','f') not null default 'm',
    -> age int not null default 18, primary key(id));

mysql> desc student;
+-------+---------------+------+-----+---------+----------------+
| Field | Type          | Null | Key | Default | Extra          |
+-------+---------------+------+-----+---------+----------------+
| id    | int(11)       | NO   | PRI | NULL    | auto_increment |
| name  | varchar(50)   | NO   |     | NULL    |                |
| sex   | enum('m','f') | NO   |     | m       |                |
| age   | int(11)       | NO   |     | 18      |                |
+-------+---------------+------+-----+---------+----------------+
mysql> insert into student values(1,'jack','m',18);
mysql> insert into student(name) values('robby');
mysql> insert into student values(1,'wf','m',20);
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

mysql> select * from student;
+----+-------+-----+-----+
| id | name  | sex | age |
+----+-------+-----+-----+
|  1 | jack  | m   |  18 |
|  2 | robby | m   |  18 |
+----+-------+-----+-----+

4)复合多列主键：多个列组合作为主键，单列值可以重复

mysql> create table service(ip varchar(15) not null,
    -> service varchar(10) not null,
    -> port int not null,
    -> protocol enum('tcp','udp') not null default 'tcp',
    -> allow enum('Y','N') default 'N',
    -> primary key(ip,service));
	
mysql> desc service;
+----------+-------------------+------+-----+---------+-------+
| Field    | Type              | Null | Key | Default | Extra |
+----------+-------------------+------+-----+---------+-------+
| ip       | varchar(15)       | NO   | PRI | NULL    |       |
| service  | varchar(10)       | NO   | PRI | NULL    |       |
| port     | int(11)           | NO   |     | NULL    |       |
| protocol | enum('tcp','udp') | NO   |     | tcp     |       |
| allow    | enum('Y','N')     | YES  |     | N       |       |
+----------+-------------------+------+-----+---------+-------+

mysql> insert into service values('192.168.1.32','http',80,'tcp','Y');
mysql> insert into service(ip,service,port) values('192.168.1.32','ftp',21);
mysql> insert into service(ip,service,port) values('192.168.1.32','http',80);
ERROR 1062 (23000): Duplicate entry '192.168.1.32-http' for key 'PRIMAR

mysql> select * from service;
+--------------+---------+------+----------+-------+
| ip           | service | port | protocol | allow |
+--------------+---------+------+----------+-------+
| 192.168.1.32 | dns     |   53 | tcp      | N     |
| 192.168.1.32 | ftp     |   21 | tcp      | N     |
| 192.168.1.32 | http    |   80 | tcp      | Y     |
+--------------+---------+------+----------+-------+

5)unique：标识该字段唯一，可以为空，一个表中可以有多个unique key

mysql> create table department(dept_id int,dept_name varchar(50) unique);

mysql> desc department;
+-----------+-------------+------+-----+---------+-------+
| Field     | Type        | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| dept_id   | int(11)     | YES  |     | NULL    |       |
| dept_name | varchar(50) | YES  | UNI | NULL    |       |
+-----------+-------------+------+-----+---------+-------+

mysql> insert into department values(1,"hr");

mysql> insert into department values(1,"it");

mysql> insert into department values(1,"hr");
ERROR 1062 (23000): Duplicate entry 'hr' for key 'dept_name'
mysql> select * from department;
+---------+-----------+
| dept_id | dept_name |
+---------+-----------+
|       1 | hr        |
|       1 | it        |
+---------+-----------+

（3）修改表

1)语法

修改表：
alter table 表名  rename 新表名
增加字段：
alter table 表名 add 字段名 数据类型[约束条件]
alter table 表名 add 字段名 数据类型[约束条件] first;
alter table 表名 add 字段名 数据类型[约束条件] after 字段名;
删除字段：
alter table 表名 drop 字段名
修改字段类型：
alter table 表名 modify 字段名 数据类型[约束条件];
alter table 表名 change 旧字段名 新字段名 新数据类型[约束条件];

2)修改表的存储引擎：

mysql> alter table t1 engine=innodb;

3)添加字段

mysql> create table t1(id int);
mysql> alter table t1 add name varchar(50) not null,add age int not null default 20;
mysql> alter table t1 add num int not null after name, add sex enum('m','f') not null default 'm' first

4)删除字段

mysql> alter table t1 drop sex;

5)修改字段类型：

mysql> alter table t1 modify age tinyint;   \注意保留原有的约束条件
mysql> alter table t1 modify age tinyint not null default 20;

mysql> alter table t1 modify id int not null primary key auto_increment;  \在字段有主键的情况下添加auto_increment这种方法不行
ERROR 1068 (42000): Multiple primary key defined
mysql> alter table t1 modify id int not null  auto_increment;   \在字段有主键的情况下添加auto_increment方式

6)添加复合主键：

mysql >alter table t2 add primary key(id,name);

7)添加主键：

mysql> alter table t1 modify id int not null primary key;  \添加主键，不建议
mysql> alter table t1 add primary key(id);

8)删除主键：如果字段有auto_increment约束，需要先删除这个约束才能在删除主键，因为自增依赖主键

mysql> alter table t1 drop primary key;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
mysql> alter table t1 modify id int not null;   \先删除主键上的自增
mysql> alter table t1 drop primary key;   \删除主键

（4）复制表

1)复制表结构+记录，key不会复制，也就是主键丶索引不会复制

mysql> create table new_t1 select * from t1;  \key不会复制，也就是主键丶索引不会复制

2)只复制表结构

mysql> create table new_t2 select * from t1 where 1=3;  //条件为假，查不到任何记录

3)复制表结构，包括key

mysql> create table new_t3 like t1;