如何通过查询在mySQL组中显示计数为零的行? [英] How to show rows with zero counts in mySQL group by query?

问题描述

我尝试寻找类似的解决方案，但对我来说似乎并没有太大帮助.我有这样一张桌子:

I tried looking around for similar solutions but it didn't seem to help much in my case. I have a table as such:

locations => |longname|name|locations|

topics    => |longname|name|tag|

，我正在尝试获取特定位置具有特定atag的长名称的数量.到目前为止，我所查询的内容也尝试强制重新排序，这就是为什么您看到location IN和FIELD的原因.但是，如果某个位置没有人要使用特定标签，则会完全从响应中删除该标签.我希望它返回0.我该怎么做?

and I am trying to get the count of longnames with a specific atag for a particular location. The query I have so far also tries to force reordering, which is why you see the location IN and FIELD. However, if there aren't any people in a location for a particular tag, it omits it entirely from the response. I'd like it to return 0. How can I do that?

select count(b.longname) as count, a.location 
 from `topics` b, locations a 
  where tag = 'atag' and a.longname = b.longname 
  and location IN ('Japan', 'France', 'Italy') 
  group by location 
  order by FIELD(location, 'Japan', 'France', 'Italy')

推荐答案

也许是这样吗? :

SELECT COUNT(t.longname) as cnt, l.location 
FROM locations l 
  LEFT JOIN topics t
    ON  t.longname = l.longname
    AND t.tag = 'atag'      
WHERE l.location IN ('Japan', 'France', 'Italy') 
GROUP BY l.location 
ORDER BY FIELD(l.location, 'Japan', 'France', 'Italy')

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