如何在MySQL命令行中给表添加一个字段?
先看一下最简单的例子,在test中,添加一个字段,字段名为birth,类型为date类型。
mysql> alter table test add column birth date;
Query OK, 0 rows affected (0.36 sec)
Records: 0 Duplicates: 0 Warnings: 0
查询一下数据,看看结果:
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)
从上面结果可以看出,插入的birth字段,默认值为空。我们再来试一下,添加一个birth1字段,设置它不允许为空。
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,这个语句不会成功的,因为我没有给他指定一个默认值。我们来看看数据:
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类型的值,设置了一个默认值:0000-00-00。下面我来直接指定一个默认值看看:
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)
看到没,将增加的birth2字段,就有一个默认值了,而且这个默认值是我们手工指定的。
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