编辑代码

mysql> CREATE DATABASE SchoolDB;
Query OK, 1 row affected (0.00 sec)

mysql> USE SchoolDB;
Database changed
 create table student(xid int primary key auto_increment,xname char(10),xnage int);
Query OK, 0 rows affected (0.02 sec)
mysql> 

mysql> DESCRIBE student;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| xid   | int(11)  | NO   | PRI | NULL    | auto_increment |
| xname | char(10) | YES  |     | NULL    |                |
| xnage | int(11)  | YES  |     | NULL    |                |
+-------+----------+------+-----+---------+----------------+
3 rows in set (0.01 sec)

mysql> alter table student add xaddress char(20) default 'Henan';
Query OK, 0 rows affected (0.04 sec)

mysql> DESCRIBE student;
+----------+----------+------+-----+---------+----------------+
| Field    | Type     | Null | Key | Default | Extra          |
+----------+----------+------+-----+---------+----------------+
| xid      | int(11)  | NO   | PRI | NULL    | auto_increment |
| xname    | char(10) | YES  |     | NULL    |                |
| xnage    | int(11)  | YES  |     | NULL    |                |
| xaddress | char(20) | YES  |     | Henan   |                |
+----------+----------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

mysql> alter table student modify xname char(10) not null nuique;
ERROR 1064 (42000): 
mysql> DESCRIBE student;
+----------+----------+------+-----+---------+----------------+
| Field    | Type     | Null | Key | Default | Extra          |
+----------+----------+------+-----+---------+----------------+
| xid      | int(11)  | NO   | PRI | NULL    | auto_increment |
| xname    | char(10) | YES  |     | NULL    |                |
| xnage    | int(11)  | YES  |     | NULL    |                |
| xaddress | char(20) | YES  |     | Henan   |                |
+----------+----------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
--------------------------------------------------------------------------
mysql>  CREATE DATABASE SchoolDB;
Query OK, 1 row affected (0.00 sec)

mysql> USE SchoolDB;
Database changed
mysql>  create table student(xid int primary key auto_increment,xname char(10),xnage int);
Query OK, 0 rows affected (0.59 sec)

mysql>  DESCRIBE student;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| xid   | int(11)  | NO   | PRI | NULL    | auto_increment |
| xname | char(10) | YES  |     | NULL    |                |
| xnage | int(11)  | YES  |     | NULL    |                |
+-------+----------+------+-----+---------+----------------+
3 rows in set (0.02 sec)

mysql> alter table student add xaddress char(20) default 'Henan';
Query OK, 0 rows affected (0.64 sec)

mysql>  DESCRIBE student;
+----------+----------+------+-----+---------+----------------+
| Field    | Type     | Null | Key | Default | Extra          |
+----------+----------+------+-----+---------+----------------+
| xid      | int(11)  | NO   | PRI | NULL    | auto_increment |
| xname    | char(10) | YES  |     | NULL    |                |
| xnage    | int(11)  | YES  |     | NULL    |                |
| xaddress | char(20) | YES  |     | Henan   |                |
+----------+----------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

mysql> alter table student modify xname char(10) not null nuique;
ERROR 1064 (42000): 
mysql> alter table student modify xname char(10) not null nuique;