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注意可转换成的数据类型只能是：
binary，char，date，datetime，decimal，signed，unsigned，time
并没有常用的
int, varchar
MySQL的类型转换函数如下：
CAST(expr AS type), CONVERT(expr,type), CONVERT(expr USING transcoding_name)
The CAST() and CONVERT() functions take a value of one type and produce a value of another type.
The type can be one of the following values:
BINARY[(N)]
CHAR[(N)]
DATE
DATETIME
DECIMAL
SIGNED [INTEGER]
TIME
UNSIGNED [INTEGER]
mysql> select cast(now() as char);
+---------------------+
| cast(now() as char) |
+---------------------+
| 2006-06-15 09:31:20 |
+---------------------+
mysql> select convert( '中文 ' using gb2312);
+------------------------------+
| convert( '中文 ' using gb2312) |
+------------------------------+
| 中文 |
+------------------------------+
1 row in set (0.06 sec)
使用日期函数date_add,date_sub就可以了,见下例：
mysql> select date_add(now(),interval 1 day);
+--------------------------------+
| date_add(now(),interval 1 day) |
+--------------------------------+
| 2006-06-16 09:36:06 |
+--------------------------------+
1 row in set (0.02 sec)
mysql> select date_add(now(), interval 1 month);
+-----------------------------------+
| date_add(now(), interval 1 month) |
+-----------------------------------+
| 2006-07-15 09:36:38 |
+-----------------------------------+
1 row in set (0.00 sec)
mysql> select date_add(now(), interval 1 minute);
+------------------------------------+
| date_add(now(), interval 1 minute) |
+------------------------------------+
| 2006-06-15 09:38:03 |
+------------------------------------+
1 row in set (0.00 sec)
1 row in set (0.05 sec)
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