4.1 问题
具体要求如下：
建表时，创建主键
在已有表里添加主键
建表时创建复合主键
删除主键
设置字段值自增长
4.2 步骤
实现此案例需要按照如下步骤进行。
步骤一：练习主键的使用
1）建表时设置PRIMARY KEY主键索引
Mysql> create database db2;
mysql> CREATE TABLE db2.biao01(
-> id int(4) PRIMARY KEY, //直接在字段定义时约束
-> name varchar(8)
-> );
Query OK, 0 rows affected (0.19 sec)
或者：
mysql> CREATE TABLE db2.biao02(
-> id int(4),
-> name varchar(8),
-> PRIMARY KEY(id) //所有字段定义完，最后指定
-> );
Query OK, 0 rows affected (0.17 sec)
在建表的时候，如果主键字段为int类型，还可以为其设置AUTO_INCREMENT自增属性，这样当添加新的表记录时，此字段的值会自动从1开始逐个增加，无需手动指定。比如，新建一个tea6表，将id列作为自增的主键字段：
mysql> CREATE TABLE db2.tea6(
-> id int(4) AUTO_INCREMENT,
-> name varchar(4) NOT NULL,
-> age int(2) NOT NULL,
-> PRIMARY KEY(id)
-> );
Query OK, 0 rows affected (0.29 sec)
2）删除现有表的PRIMARY KEY主键索引
如果要移除某个表的PRIMARY KEY约束，需要通过ALTER TABLE指令修改。比如，以下操作将清除biao01表的主键索引。
清除前（主键为id）：
mysql> DESC db2.biao01;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(4) | NO | PRI | NULL | |
| name | varchar(8) | YES | | NULL | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
清除操作：
mysql> ALTER TABLE db2.biao01 DROP PRIMARY KEY;
Query OK, 0 rows affected (0.49 sec)
Records: 0 Duplicates: 0 Warnings: 0
清除后（无主键）：
mysql> DESC db2.biao01;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(4) | NO | | NULL | |
| name | varchar(8) | YES | | NULL | |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
当尝试删除tea6表的主键时，会出现异常：
mysql> ALTER TABLE tea6 DROP PRIMARY KEY;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
这是因为tea6表的主键字段id具有AUTO_INCREMNET自增属性，提示这种字段必须作为主键存在，因此若要清除此主键必须先清除自增属性——修改id列的字段定义：
mysql> ALTER TABLE tea6 MODIFY id int(4) NOT NULL;
Query OK, 0 rows affected (0.75 sec)
Records: 0 Duplicates: 0 Warnings: 0
然后再清除主键属性就OK了：
mysql> ALTER TABLE tea6 DROP PRIMARY KEY; //清除主键
Query OK, 0 rows affected (0.39 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc tea6; //确认清除结果
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(4) | NO | | NULL | |
| name | varchar(4) | NO | | NULL | |
| age | int(2) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
3）为现有表添加PRIMARY KEY主键索引
重新为tea6表指定主键字段，仍然使用id列：
mysql> ALTER TABLE tea6 ADD PRIMARY KEY(id); //设置主键字段
Query OK, 0 rows affected (0.35 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tea6; //确认设置结果
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id | int(4) | NO | PRI | NULL | |
| name | varchar(4) | NO | | NULL | |
| age | int(2) | NO | | NULL | |
+-------+------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
4）建表时创建复合主键
mysql>
mysql> create table db2.t6(
-> class char(7),
-> name char(15),
-> pay enum("yes","no") default "no",
-> primary key(class,name,pay) //指定多个字段一起做主键
-> );
Query OK, 0 rows affected (0.04 sec)
mysql> desc db2.t6;
+-------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| class | char(7) | NO | PRI | | |
| name | char(15) | NO | PRI | | |
| pay | enum('yes','no') | NO | PRI | no | |
+-------+------------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
mysql>
