MySQL Left Join子选择 [英] MySQL Left Join Subselect

问题描述

我有一个简单的语言表/模板ID:

I have a simple table of languages / template id's:

语言|模板

en，t1
zh_CN，t1
au，t2
ge，t3
zh_CN，t1
ge，t2
ge，t3

en, t1
en, t1
au, t2
ge, t3
en, t1
ge, t2
ge, t3

模板始终为t1，t2或t3.总共有3种语言:en，au，ge.

Template is always either t1,t2, or t3. In total there are 3 languages: en, au, ge.

表中还有很多信息，我仅显示与此问题相关的信息，我将使用数据进行图形绘制，因此需要以以下格式返回:

There is lots more information in the table, I am just showing what is relevant to this question, I will be using the data for graphing and so need it returning in this format:

en，t1，3
en，t2，0
en，t3，0
au，t1，0
au，t2，1
au，t3，0
ge，t1，0
ge，t2、1
ge，t3、2

en, t1, 3
en, t2, 0
en, t3, 0
au, t1, 0
au, t2, 1
au, t3, 0
ge, t1, 0
ge, t2, 1
ge, t3, 2

这将计算每种语言中出现的模板数量.但是，如果表中没有该特定语言的模板ID，那么我的问题是返回零计数.

This counts however many template occurrences there are in each language. But, the problem I have is returning a zero count if there are no template id's for that particular language in the table.

我当时想需要在模板ID上进行某种左联接子选择，以确保每种语言都返回3个模板ID?

I was thinking it would need some sort of left join sub select on the template id to make sure the 3 template id's are returned for each language?

推荐答案

也许有更好的方法，我还没有在MySQL中对其进行测试，但是以下在SQL Server 2005中起作用:

There might be a better way of doing this, and I haven't tested it in MySQL, but the following works in SQL Server 2005:

Select a.language, b.template, count (c.template) as combo_count
from
(select distinct language from tablename) as a
inner join (select distinct template from tablename) as b on 1 < 2 /* this could be cross join, same thing. */
left outer join tablename c on c.language = a.language and c.template = b.template
group by a.language, b.template
order by 1, 2

以下是带有示例数据的结果:

Here are the results with your sample data:

au  t1  0
au  t2  1
au  t3  0
en  t1  3
en  t2  0
en  t3  0
ge  t1  0
ge  t2  1
ge  t3  2

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