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MySQL的自增语句大家应该都很熟悉 也很简单
update `info` set `comments` = `comments`+1 WHERE `id` = 32
这样就可以了，但是有时候我们会涉及到做减法，
例如：文章的评论数，在删除或者锁定了一条评论之后需要对该文章总评论数减一
comments smallint(5) unsigned 文章评论总数统计字段 无符号即 0 ~ 65535 之间的数值
1. 通常情况下是可以类似上面自增的方法 把 +号 改成 -号 就行了，但问题是如果当前 comments 统计数值为 0 时 再做减法将会变成该字段类型的最大数值 65535
update `info` set `comments` = `comments`-1 WHERE `id` = 32
2. 为避免这个问题一般的想法只能是先根据 id 主键查询出文章 comments 统计字段值，再通过PHP做减法，然后再 update 一次，前后总共需要执行两次SQL命令
今天google查了下没找到这方面的资料，看了看MySQL的语法函数等等。。。试了下面的语句可以直接一条语句完成，也就是加个 if 判断，如下示例：
update `info` set `comments` = IF(`comments`
默认comments为0时， comments-1 = 65535；但测试了下 如果直接 判断 comments-1=65535 好像不行，不知道什么原因,对这个不是很熟悉不知道是不是 这里的 if 不支持 = 号，但是 comments-1 >= 65535 可以成立，于是当 comments 为 0 时，IF(`comments`-1>=65535,0,`comments`-1) 将返回 0提示：最大数值 65535 是 smallint 无符号状态下的最大值，其他字段类型请进行相应调整
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2014/02/03 补充：刚开始是这么写的，后来发现太笨了，稍微改下：
update `info` set `comments` = IF(`comments`
要减x，就判断是否小于xbitsCN.com
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