MySQL关于分组的混淆。为什么会这样?
为什么这三个问题如此不同?根据MySQL文档,它们不都应该做相同的事情吗MySQL关于分组的混淆。为什么会这样?,mysql,sql,Mysql,Sql,为什么这三个问题如此不同?根据MySQL文档,它们不都应该做相同的事情吗 select cc, max(c) as "Most Official Billingual Count" from ( select CountryCode as cc, count(*) as c from countrylanguage where isofficial = "T" group by cc ) t group by cc 以以下格式返回多行:CountryCode官方语言计数bla 但是如果
select cc, max(c) as "Most Official Billingual Count" from (
select CountryCode as cc, count(*) as c from countrylanguage where isofficial = "T"
group by cc
) t group by cc
以以下格式返回多行:CountryCode官方语言计数bla
但是如果我删除一组
select cc, max(c) as "Most Official Billingual Count" from (
select CountryCode as cc, count(*) as c from countrylanguage where isofficial = "T"
) t group by cc
它返回第一个国家代码以及总计数:
ABW 238
这是:
select cc, max(c) as "Most Official Billingual Count" from (
select CountryCode as cc, count(*) as c from countrylanguage where isofficial = "T"
group by cc
) t
将返回如下内容:ABW 4
我对这整群人的事感到很困惑
内部查询
select CountryCode as cc, count(*) as c from countrylanguage where isofficial = "T
是否应返回所有国家代码并计算重复的国家代码?对吗?但它不能做到这一点。似乎我必须执行第一个队列才能获得所需的效果,这让我非常困惑您的子查询:
select CountryCode as cc, count(*) as c from countrylanguage where isofficial = "T"
除非您没有按照相应的标准对记录进行分组,否则没有意义。您需要对记录进行分组,因为WHERE
子句仅指示选择标准。要了解groupby
,请参见此(DBMS是Oracle,但无所谓)。此外(我更喜欢阅读),你可以找到许多关于这方面的文章,例如
因此,如果您需要在计数之间找到
MAX
,您需要首先通过子查询中的groupby
组合您的组。谢谢,我明天醒来时会看视频