一、对已存在的表添加主键约束

mysql> alter table dept add primary key(deptno);
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> select * from dept;
+--------+--------------+
| deptno | dept_name    |
+--------+--------------+
|      1 | zhaoyang     |
|      2 | zhouwenqiang |
|      3 | 实施部       |
|      4 | 产品部       |
+--------+--------------+
4 rows in set (0.00 sec)

mysql>

2. 第二种格式：alter table 表名 add constraint 主键约束的名字 primary key(列名);

mysql> update employees set empno = 1 where ename=
    -> 'zhaoyang';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from employees;
+-------+--------------+------+------------+------------+---------+--------+
| empno | ename        | age  | birth      | hiredate   | sal     | deptno |
+-------+--------------+------+------------+------------+---------+--------+
|     1 | zhaoyang     |   24 | 1993-07-17 | 2021-10-18 | 5000.00 |      1 |
|  NULL | chenqian     |   21 | 1996-05-12 | 2021-10-21 | 3001.00 |      1 |
|  NULL | dengdesheng  |   19 | 1999-02-25 | 2021-11-15 | 4000.00 |      1 |
|  NULL | zhouwenqiang |   21 | 2001-01-01 | 2021-08-19 | 7000.00 |      2 |
+-------+--------------+------+------------+------------+---------+--------+
4 rows in set (0.00 sec)

mysql> update employees set empno=2 where ename='chenqian';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> update employees set empno=3 where ename='dengdesheng';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> alter table employees add constraint pk_emp primary key(empno);
ERROR 1138 (22004): Invalid use of NULL value
mysql> update employees set empno=4 where ename='zhouwenqiang';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> alter table employees add constraint pk_emp primary key(empno);
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0. 

第三种格式：alter table 表名 modify 列名 数据类型 primary key；

mysql> alter table employees modify empno int primary key;
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0

删除主键约束：

mysql> alter table employees drop primary key;
Query OK, 4 rows affected (0.05 sec)
Records: 4  Duplicates: 0  Warnings: 0

二、在创建表的时候同时创建主键约束

第一种格式：

create table 表名(

　　列名1  数据类型 primary key,

　　列名2 数据类型 );

mysql> create table if not exists location(
    ->  localno int primary key,
    ->  localname varchar(30));
Query OK, 0 rows affected (0.03 sec)

第二种格式：

create table 表名(

　　列名1 数据类型,

　　列名2 数据类型,

　　constraint  主键约束的名字  primary key(列名1) );

mysql> create table if not exists student_info(
    ->  number int,
    ->  name   varchar(5),
    ->  sex     enum('M','F'),
    ->  id_number       char(18),
    ->  department      varchar(30),
    ->  major varchar(30),
    ->  enrollment_time date,
    ->  constraint pk_student_info primary key(number));
Query OK, 0 rows affected, 1 warning (0.01 sec)

第三种格式：

create table 表名(

　　列名1 数据类型,

　　列名2 数据类型,

　　primary key(列名1) );

mysql> create table if not exists student_info(
    ->  number int,
    ->  name   varchar(5),
    ->  sex     enum('M','F'),
    ->  id_number       char(18),
    ->  department      varchar(30),
    ->  major varchar(30),
    ->  enrollment_time date,
    ->  primary key(number));
Query OK, 0 rows affected (0.03 sec)