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关于Group By的用法，实现分组统计

以前始终不知道这个SQL语句啥意思，太懒了没仔细看，当自己遇到麻烦了才去了解group by是什么作用。
例子：其中一个日记表，假设只有两个字段，一个日期，一个标题。日期是可重复的，也就是说一天可以写N条日记。
我现在要统计某天的日记，当然很简单

select count(*) from 日记表 where 发表日期="2010-4-8"

如果我要依次列出，日期，当天发贴数呢。没出现的日期不要列，有的可能会想到一个循环，把所有时间来where。
这样很费劲的，而且会把不该列的日期列出来。其实一条语句就可以解决。

SELECT 发表日期,count(*) FROM 日记表 GROUP BY 发表日期 Order By 发表日期 ASC"

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