1.单字段主键
create table 表名 (属性名 数据类型 primary key,...);
for example :create table t_dept(
             deptno int primary key,
             dname varchar(20),
             loc varchar(40));
2.如果相关字段PK设置一个名字，可以执行sql语句constraint,如：
 create table t_dept(
       deptno int,
       dname varchar(20),
       loc varchar(40),
       constraint pk_dname primary key(dname));
3.多字段主键
 create table 表名 （
        属性名 数据类型，...
        [constraint 约束名] primary key (属性名，属性名...));
for example: create table t_dept(
              deptno int,
              dname varchar(20),
              loc varchar(20),
              constraint pk_dname_deptno primary key(deptno,dname；

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4. 例子；创建表时建立主键索引
mysql> create table student (
    -> id int(4) not null auto_increment,
    -> name char(20) not null,
    -> age tinyint(2) not null default '0',
    -> dept varchar(16) default null,
    -> primary key(id),
    -> KEY index_name(name)
    -> );
mysql> desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(4)      | NO   | PRI | NULL    | auto_increment |
| name  | char(20)    | NO   | MUL | NULL    |                |
| age   | tinyint(2)  | NO   |     | 0       |                |
| dept  | varchar(16) | YES  |     | NULL    |                |
+-------+-------------+------+-----+---------+----------------+
-------------------------------------------------------------------
5.建表后通过alter命令增加主键索引（一般不这样做）
 mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(4)      | NO   |     | NULL    |       |
| name  | char(20)    | NO   |     | NULL    |       |
| age   | tinyint(2)  | NO   |     | 0       |       |
| dept  | varchar(16) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table student change id id int primary key auto_increment;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name  | char(20)    | NO   |     | NULL    |                |
| age   | tinyint(2)  | NO   |     | 0       |                |
| dept  | varchar(16) | YES  |     | NULL    |                |
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)