mysql字符串的隐式转换导致查询异常的问题_MySQL
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mysql字符串的隐式转换导致查询异常的问题 先看问题:01CREATE TABLE ceshi (id int(10) unsigned NOT NULL AUTO_INCREMENT,title varchar(1000) NOT NULL DEFAULT '0',PRIMARY KEY(id));02 INSERT INTO ceshi(title) VALUES('你好啊');03INSERT INTO ceshi(title) VALUES('hello');04INSERT INTO ceshi(title) VALUES('0');05INSERT INTO ceshi(title) VALUES('1');06 07mysql> select * from ceshi where title=0;08+----+--------+09| id | title |10+----+--------+11| 1 | 你好啊 |12| 2 | hello |13| 3 | 0 |14+----+--------+15 3 rows in set (0.00 sec)16 17mysql> select * from ceshi where title='0';18+----+-------+19| id | title |20+----+-------+21| 3 | 0 |22+----+-------+231 row in set (0.00 sec)解释: 字段是 varchar 型,执行 select * from ceshi where title=0;的时候,要把 title 字段做类型转换,换为 int 型来比较。对于文字或字母的值,转换为 int 型就是 0,所以 abc啊,中文什么的就转为 0 了;如果是数字的值,则转换为相应的数据,所以 ‘0’转换为 0,'1' 转换为 1,这样,就除了 '1' 这条记录外,都满足 = 0 的条件。 第二个语句是与字符 '0' 比较,这个就不需要做类型转换,原样的值进行比较,因此只有 '0' 是完全相符的记录。 另:但是为什么title=0不转化成字符0来比较呢?因为title字段就是varchar类型。 这个可以这样理解,常量 0 是你自己写的,说明你的“本意”就是想用数字 0 来比较,如果说我们写的常量值都被转换了,这程序岂不是让人很难捉摸了? 这个转换你可以通过 0 和 '0' 来表示想用数值还是字符来比较,这样给使用者带来很大的自由。如果以 title 的类型来定的话,那你想用数值来比较,那不是很麻烦了,还要 cast() 之类之类的强制转化,显然不如现有的规则来的方便。
http://topic.csdn.net/u/20120921/11/b5166478-386f-4c63-90f6-a2bcab141170.html
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