修改表的单列或者多列

mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(10) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | MUL | NULL | |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)

mysql> ALTER TABLE userS1 ADD age TINYINT UNSIGNED NOT NULL DEFAULT 10;
Query OK, 0 rows affected (0.75 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(10) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | MUL | NULL | |
| age | tinyint(3) unsigned | NO | | 10 | |
+----------+----------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

mysql> ALTER TABLE users1 ADD password VARCHAR(32) NOT NULL AFTER username;
Query OK, 0 rows affected (0.59 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(10) | NO | | NULL | |
| password | varchar(32) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | MUL | NULL | |
| age | tinyint(3) unsigned | NO | | 10 | |
+----------+----------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> ALTER TABLE users1 ADD truename VARCHAR(20) NOT NULL FIRST;
Query OK, 0 rows affected (0.61 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| truename | varchar(20) | NO | | NULL | |
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(10) | NO | | NULL | |
| password | varchar(32) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | MUL | NULL | |
| age | tinyint(3) unsigned | NO | | 10 | |
+----------+----------------------+------+-----+---------+----------------+
6 rows in set (0.00 sec)

mysql> ALTER TABLE users1 DROP truename;
Query OK, 0 rows affected (0.63 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(10) | NO | | NULL | |
| password | varchar(32) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | MUL | NULL | |
| age | tinyint(3) unsigned | NO | | 10 | |
+----------+----------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> ALTER TABLE users1 DROP password,DROP age;
Query OK, 0 rows affected (0.62 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(10) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | MUL | NULL | |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)