MySQL数据库(四)：约束条件
安装环境： 操作系统版本：RHEL 6.5 版本：MYSQL 5.5 约束条件的作用： 限制如何给字段赋值，创建表的时候如果没有设置的话，就是mysql建表的默认设置包括表结构的后四列。 1、NULL 是否允许空值 在不设置的时候，默认允许字段值为空
mysql> desc yueshu;
+-------+--------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------------+------+-----+---------+-------+
| name | varchar(20) | NO | PRI | NULL | |
| age | int(2) | YES | | 10 | |
| sex | enum('boy','girl') | YES | | boy | |
+-------+--------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)查询字段值是否为空
mysql> select * from yueshu where age is null;
+------+------+------+
| name | age | sex |
+------+------+------+
| | NULL | boy |
+------+------+------+
1 row in set (0.00 sec)mysql> insert into yueshu(age) values(null);
Query OK, 1 row affected, 1 warning (0.00 sec)查询字段的值
mysql> select * from yueshu where sex="boy";
+------+------+------+
| name | age | sex |
+------+------+------+
| | NULL | boy |
+------+------+------+
1 row in set (0.00 sec)2、NOT NULL 不允许为空
mysql> desc yueshu;
+-------+--------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------------+------+-----+---------+-------+
| name | varchar(20) | NO | PRI | NULL | |
| age | int(2) | YES | | 10 | |
| sex | enum('boy','girl') | YES | | boy | |
+-------+--------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> insert into yueshu(name) values(null);
ERROR 1048 (23000): Column 'name' cannot be null3、KEY 索引类型(是否为主键) 4、DEFAULT 设置默认值,缺省为NULL.向表中插入新的记录时，若不给字段赋值，就使用默认值给字段赋值。建表时，没有设置字段的默认值，mysql吧字段的默认值设置为空
例子：
mysql> create table yueshu(
-> name varchar(20) not null,
-> age int(2) default 10,
-> sex enum("boy","girl") default "boy",
-> primary key(name)
-> );举报
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