解析MySQL数据库中执行SQL语句的注意点_MySQL
bitsCN.com 在执行查找命令时的大小写问题
默认情况下，MySQL在执行查找时不区分字母的大小写。当你在查找时使用like a%时， 默认将返回以a或者A开头的所有列值，若要在查找时区分大小写，则需要使用collate运算符来实现，具体例子如下：
如果想让某列在查找时始终保持区分字母大小写，则需要用case sensitive或binary collation来定义列的特性。
当进行简单比较运算，如>=, >, =,
和空值NULL相关的问题
空值的概念比较容易混淆，有人认为空值NULL和空 字符是一会时，其实这样认为是错误的，举例来说，
上面两条语句有完全不同的意义和结果。第一句表示，向表中插入的记录中的某人的电话号码NULL，意思是还不知道这 条记录中的人的电话号码，而第二句向表中插入空字符，就意味着已经知道表中这条记录中的人的电话号码，这个电话号码 是,也可以确定此人没有正常的电话号码。
帮助处理空值NULL的方法，主要通过执行is null运算判断，is not null运算 判断，和IFNULL()函数运算来实现。在SQL中，任何与NULL发生运算关系的结果都不可能为真(即永远是假)。为了寻找NULL值 ，必须执行is null测试运算。
可以在MyISAM, InnoDB, BDB或MEMORY等存储类型的表列中含有空值时添加index，但该 index必须声明成not null,这样之后将不可以往该列中输入空值。
当使用load data infile语句载入数据时，空值NULL将 被替换成。使用DISTINCT, GROUP BY, 或ORDER BY时所有空值将被视为等同。像COUNT(), MIN(), SUM()等概要总结函数将忽略空值NULL。
但对一些特殊类型的列来说，比如timestamp类型和具有auto_increment属性的列：前者插入NULL是实际将插入当前时间值，向后者插入NULL时将是下个正整数序列。
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