表的约束

约束条件 说明
PRIMARY KEY 主键约束，用于唯一标识对应的记录
FOREIGN KEY 外键约束
NOT NULL 非空约束
UNIQUE 唯一性约束
DEFAULT 默认值约束，用于设置字段的默认值

多字段主键指的是多个字段组合而成的主键(PRIMARY KEY (字段名1,字段名2,……字段名n) )
mysql> CREATE TABLE yyqx_1
->
-> (
-> stu_id INT,
-> course_id INT,
-> grade FLOAT,
-> PRIMARY KEY(stu_id,course_id)
-> );
Query OK, 0 rows affected

mysql> DESC yyqx_1;
+-----------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| stu_id | int(11) | NO | PRI | 0 | |
| course_id | int(11) | NO | PRI | 0 | |
| grade | float | YES | | NULL | |
+-----------+---------+------+-----+---------+-------+
3 rows in set

使用AUTO_INCREMENT设置表字段值自动增加(字段名 数据类型 AUTO_INCREMENT; )
mysql> CREATE TABLE yyqx_2
-> (
-> id INT PRIMARY KEY AUTO_INCREMENT,
-> stu_id INT UNIQUE,
-> grade FLOAT DEFAULT 0
-> );
Query OK, 0 rows affected

mysql> DESC yyqx_2;
+--------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| stu_id | int(11) | YES | UNI | NULL | |
| grade | float | YES | | 0 | |
+--------+---------+------+-----+---------+----------------+
3 rows in set

mysql> CREATE TABLE yyqx_3
-> (
-> id INT PRIMARY KEY AUTO_INCREMENT,
-> stu_id INT UNIQUE,
-> grade FLOAT
-> );
Query OK, 0 rows affected

mysql> DESC yyqx_3;
+--------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| stu_id | int(11) | YES | UNI | NULL | |
| grade | float | YES | | NULL | |
+--------+---------+------+-----+---------+----------------+
3 rows in set

表的约束

原文：https://www.cnblogs.com/bjyxszdxzwyb/p/11747431.html