平时都是使用myssql的我，最近不得不开始学习mssql了。
在mysql下：
select author, Datepart(hour, created) as hh from comments
where created>='2017-08-01 0:00:00'
and title like "穿越诗歌"
and permlink not like 're-%'
group by Datepart(hour, created)
这是一点问题也没有的。可是在mssql下，系统提示：
Error 8120: Column 'author' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
虽然我的英语不咋的，但是计算英语还是能看懂大概的。这句的意思是：author这个字段在查询中无效，因为它既不用于聚合函数，也不属于组子句的字段。
我蒙了呀，难道我如果查询10个字段，都要按照分个组吗？
后来我冷静的思考了一下。分组以后，每组的author值有很多，那么这句话应该查出来一个哪一条记录了。所以要用count max min等聚合函数聚合一下才行。
在mysql中，分组以后如果没有指定，就会自动把第一条记录的author值查询出来。
事实上呢，这个含有穿越诗歌的文章都是同一个作者写的，选哪一条记录都一样。
可以看出，mssql很严谨，在语法上就不通过了，这样的句子是不能执行的。 mssql的确实是比mysql要严谨呀。
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