《Mysql實(shí)例MySQL命令行中給表添加一個字段（字段名、是否為空、默認(rèn)值）》要點(diǎn)：
本文介紹了Mysql實(shí)例MySQL命令行中給表添加一個字段（字段名、是否為空、默認(rèn)值），希望對您有用。如果有疑問，可以聯(lián)系我們。

MYSQL應(yīng)用先看一下最簡單的例子,在test中,添加一個字段,字段名為birth,類型為date類型.

mysql> alter table test add column birth date;
Query OK, 0 rows affected (0.36 sec)
Records: 0? Duplicates: 0? Warnings: 0

查詢一下數(shù)據(jù),看看結(jié)果：

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)

從上面結(jié)果可以看出,插入的birth字段,默認(rèn)值為空.我們再來試一下,添加一個birth1字段,設(shè)置它不允許為空.

mysql> alter table test add column birth1 date not null;
Query OK, 0 rows affected (0.16 sec)
Records: 0? Duplicates: 0? Warnings: 0

居然執(zhí)行成功了!?意外了!我原來以為,這個語句不會成功的,因?yàn)槲覜]有給他指定一個默認(rèn)值.我們來看看數(shù)據(jù)：

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,系統(tǒng)自動將date類型的值,設(shè)置了一個默認(rèn)值：0000-00-00.下面我來直接指定一個默認(rèn)值看看：

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?jīng)],將增加的birth2字段,就有一個默認(rèn)值了,而且這個默認(rèn)值是我們手工指定的.

關(guān)于MySQL中給表添加一個字段,本文就介紹這么多,希望對大家有所幫助,謝謝!

轉(zhuǎn)載請注明本頁網(wǎng)址：
http://www.fzlkiss.com/jiaocheng/3698.html