MySQL Left Join Subselect

问题

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

language | template

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

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

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

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.

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?

回答1:

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

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

回答2:

  Select a.language, a.template, Count(*) count
  From (Select Distinct language, template From table) a
     Left Join table b
         On b.language = a.language
             And b.template = b.template
  Group By a.language, a.template

回答3:

What you need is two tables that list the possible values for language and template.

CREATE TABLE language (...) AS SELECT DISTINCT language FROM your_table;
CREATE TABLE template (...) AS SELECT DISTINCT template FROM your_table;

Then you can do something like this:

SELECT l.language, t.template, SUM(CASE WHEN yours.language IS NULL THEN 0 ELSE 1 END) count
FROM language l CROSS JOIN template t
LEFT OUTER JOIN your_table yours ON l.language = yours.language AND t.template = yours.template
GROUP BY l.language, t.template;

来源：https://stackoverflow.com/questions/7520441/mysql-left-join-subselect

标签

mysql

left-join

subquery