从一组选定的2个统一列中计数实例

Question

假设我有一张名为“比赛”的表格，我在这里保存了来自足球比赛的2支球队。

------------------------- 
| home_club | away_club | 
------------------------- 
|   |   | 
------------------------- 

而且我有一个返回所有的俱乐部从该表的查询，无论是主场还是客场俱乐部通过UNION：

SELECT home_club AS clubs FROM matches UNION SELECT away_club FROM matches 

现在我有所谓的“俱乐部”一个结果集，我希望来计算每个出现在“匹配”表中的次数。我如何去做这件事？

Answer 1

如果你想知道有多少次，每次出现在matches表，那么你需要在子查询中摆脱union的。 union将删除重复项。

这里是你怎么能够让一个计数：

select club, count(*) as NumAppears 
from (SELECT home_club AS club FROM matches 
     UNION ALL 
     SELECT away_club FROM matches 
    ) m 
group by club; 

注意UNION替换UNION ALL。

Answer 2

尝试以下

select sum(count) as Matches, Club from 
(select count(*) as count, home_club as Club from matches group by home_club 
union all 
select count(*) as count, away_club as Club from matches group by away_club) a 
group by a.clubs 

Answer 3

SELECT 
    M.clubs, 
    COUNT(M.clubs) [count] 
FROM 
    (
     SELECT 
      home_club AS clubs 
     FROM 
      matches 

     UNION ALL 

     SELECT 
      away_club 
     FROM 
      matches 
    ) M 
GROUP BY 
    M.clubs