计算 MySQL 中的唯一组合

Question

I want to count number of matches between two teams.

select home.home_team, home.away_team, (home_matches_against+away_matches_against) as matches
from
(select home_team, away_team, count(home_team) as home_matches_against
from results
group by home_team, away_team) as home
join
(select away_team, home_team, count(away_team) as away_matches_against
from results
group by away_team, home_team) as away
on home.home_team=away.away_team and home.away_team=away.home_team

output:

As you can see count of matches between tow teams are duplicated.

My desired result is for example:

home team  away _team  matches
England    Scotland    117
Wales      Scotland    106 

and etc. I dont want count of matches to be duplicated

Answer 1

您可以添加WHERE子句：

WHERE home.home_team < home.away_team;

Answer 2

You could use LEAST and GREATEST. Something like:

SELECT t1.* 
FROM ( SELECT LEAST(home_team, away_team) AS home_team,
              GREATEST(home_team, away_team) AS away_team,
              matches
       FROM results
      ) AS t1
 GROUP BY t1.home_team, t1.away_team,t1.matches;

https://www.db-fiddle.com/f/uLPPE1DoKjKYBoSXfKahJN/21