MySQL建表设置默认值的取值范围

create table student(
id int not null  primary key auto_increment,
name varchar(32) not null,
sex varchar(8) default('男'), #设置默认值
age int not null check (age between 10 and 50), #设置取值范围
class_id int not null references class(id),
stu_num varchar(32),
constraint stu_u unique(stu_num)
);

# 格式模板
<字段名> <数据类型> DEFAULT <默认值>

# 示例
mysql> CREATE TABLE `test_tb` (
    ->   `id` int NOT NULL AUTO_INCREMENT,
    ->   `col1` varchar(50) not null DEFAULT 'a',
    ->   `col2` int not null DEFAULT 1,
    ->   PRIMARY KEY (`id`)
    -> ) ENGINE=InnoDB  DEFAULT CHARSET=utf8;
Query OK, 0 rows affected (0.06 sec)

mysql> desc test_tb;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| col1  | varchar(50) | NO   |     | a       |                |
| col2  | int(11)     | NO   |     | 1       |                |
+-------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> insert into test_tb (col1) values ('fdg');
Query OK, 1 row affected (0.01 sec)

mysql> insert into test_tb (col2) values (2);
Query OK, 1 row affected (0.03 sec)

mysql> select * from test_tb;
+----+------+------+
| id | col1 | col2 |
+----+------+------+
|  1 | fdg  |    1 |
|  2 | a    |    2 |
+----+------+------+
2 rows in set (0.00 sec)

# 添加新字段 并设置默认值
alter table `test_tb` add column `col3` varchar(20) not null DEFAULT 'abc';

# 修改原有默认值
alter table `test_tb` alter column `col3` set default '3a';
alter table `test_tb` change column `col3` `col3` varchar(20) not null DEFAULT '3b';
alter table `test_tb` MODIFY column `col3` varchar(20) not null DEFAULT '3c';

# 删除原有默认值
alter table `test_tb` alter column `col3` drop default;

# 增加默认值(和修改类似)
alter table `test_tb` alter column `col3` set default '3aa';

mysql> alter table `test_tb` add column `col4` varchar(20) DEFAULT '4a';
Query OK, 0 rows affected (0.12 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>  desc test_tb;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| col1  | varchar(50) | NO   |     | a       |                |
| col2  | int(11)     | NO   |     | 1       |                |
| col3  | varchar(20) | NO   |     | 3aa     |                |
| col4  | varchar(20) | YES  |     | 4a      |                |
+-------+-------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

CREATE TABLE `default_tb` (
  `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '自增主键',
  ...
  `country` varchar(50) not null DEFAULT '中国',
  `col_status` tinyint not null DEFAULT 1 COMMENT '1:代表啥 2:代表啥...',
  `col_time` datetime NOT NULL DEFAULT '2020-10-01 00:00:00' COMMENT '什么时间',
  `is_deleted` tinyint not null DEFAULT 0 COMMENT '0:未删除 1:删除',
  `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
  `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;