标签：ted 条件 记录 mysql mysq def hellip esc tab
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 exam_1
->
-> (
-> stu_id INT,
-> course_id INT,
-> grade FLOAT,
-> PRIMARY KEY(stu_id,course_id)
-> );
Query OK, 0 rows affected
mysql> DESC exam_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 exam_2
-> (
-> id INT PRIMARY KEY AUTO_INCREMENT,
-> stu_id INT UNIQUE,
-> grade FLOAT DEFAULT 0
-> );
Query OK, 0 rows affected
mysql> DESC exam_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 exam_3
-> (
-> id INT PRIMARY KEY AUTO_INCREMENT,
-> stu_id INT UNIQUE,
-> grade FLOAT
-> );
Query OK, 0 rows affected
mysql> DESC exam_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
标签：ted 条件 记录 mysql mysq def hellip esc tab
原文地址：https://www.cnblogs.com/cmja/p/11747372.html
