责任编辑如是说MySQL资料库采用SQL句子来在原有的附注，加进两个捷伊表头，主要包括增设表头的与否为空、缺省等。
先看呵呵最简单的范例，在test中，加进两个表头，表头名为birth，类别为date类别。
mysql> alter table test add column birth date;
Query OK, 0 rows affected (0.36 sec)
Records: 0? Duplicates: 0? Warnings: 0
查阅呵呵统计数据，看一看结论：
mysql> select * from test;
+------+--------+----------------------------------+------------+-------+
| t_id | t_name | t_password | t_birth? | birth |
+------+--------+----------------------------------+------------+-------+
|? 1 | name1? | 12345678901234567890123456789012 | NULL | NULL? |
|? 2 | name2? | 12345678901234567890123456789012 | 2013-01-01 | NULL? |
+------+--------+----------------------------------+------------+-------+
2 rows in set (0.00 sec)
从上面结论可以看出，插入的birth表头，缺省为空。我们再来试呵呵，加进两个birth1表头，增设它不允许为空。
mysql> alter table test add column birth1 date not null;
Query OK, 0 rows affected (0.16 sec)
Records: 0? Duplicates: 0? Warnings: 0
居然执行成功了！？意外了！我原来以为，这个句子不会成功的，因为我没有给他指定两个缺省。我们来看一看统计数据：
mysql> select * from test;
+------+--------+----------------------------------+------------+-------+------------+
| t_id | t_name | t_password | t_birth? | birth | birth1 |
+------+--------+----------------------------------+------------+-------+------------+
|? 1 | name1? | 12345678901234567890123456789012 | NULL | NULL? | 0000-00-00 |
|? 2 | name2? | 12345678901234567890123456789012 | 2013-01-01 | NULL? | 0000-00-00 |
+------+--------+----------------------------------+------------+-------+------------+
2 rows in set (0.00 sec)
哦，明白了，系统自动将date类别的值，增设了两个缺省：0000-00-00。下面我来直接指定两个缺省看一看：
mysql> alter table test add column birth2 date default 2013-1-1;
Query OK, 0 rows affected (0.28 sec)
Records: 0? Duplicates: 0? Warnings: 0
mysql> select * from test;
+------+--------+----------------------------------+------------+-------+------------+------------+
| t_id | t_name | t_password | t_birth? | birth | birth1 | birth2 |
+------+--------+----------------------------------+------------+-------+------------+------------+
|? 1 | name1? | 12345678901234567890123456789012 | NULL | NULL? | 0000-00-00 | 2013-01-01 |
|? 2 | name2? | 12345678901234567890123456789012 | 2013-01-01 | NULL? | 0000-00-00 | 2013-01-01 |
+------+--------+----------------------------------+------------+-------+------------+------------+
2 rows in set (0.00 sec)
看到没，将增加的birth2表头，就有两个缺省了，而且这个缺省是我们手工指定的。
