MYSQL笔记：约束条件

主键约束

不能为空，值必须是不同的（唯一性）
一个表只能修饰一个主键

PRIMARY KEY

自增约束

AUTO_INCREMENT

唯一键约束

可以为空

unique

非空约束

not null

默认值约束

default

外键约束

foreign key

外键，存储函数，存储过程，触发器，在后端用的不多，限制逻辑代码逻辑是由mysql本身控制的。

例子

CREATE TABLE user(
id INT unsigned PRIMARY KEY AUTO_INCREMENT COMMENT ' 用户id',
nickname varchar(50) UNIQUE NOT NULL COMMENT'用户名称',
age TINYINT UNSIGNED NOT NULL  DEFAULT 18,
sex ENUM('male','famale'));

mysql> show table;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| user           |
+----------------+
1 row in set (0.00 sec)

mysql> desc user;
+----------+-----------------------+------+-----+---------+----------------+
| Field    | Type                  | Null | Key | Default | Extra          |
+----------+-----------------------+------+-----+---------+----------------+
| id       | int(10) unsigned      | NO   | PRI | NULL    | auto_increment |
| nickname | varchar(50)           | NO   | UNI | NULL    |                |
| age      | tinyint(3) unsigned   | NO   |     | 18      |                |
| sex      | enum('male','famale') | YES  |     | NULL    |                |
+----------+-----------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)