Question

I’m trying to write a sql query that shows how often two teams have played against each other.

Id | Team1 | Team2 | Date 
1  |   A   |   B   | 25/5/11
2  |   B   |   A   | 26/5/11
3  |   A   |   C   | 27/5/11
4  |   C   |   B   | 28/5/11
5  |   A   |   B   | 28/5/11

result should be:

A vs B => 3
A vs C => 1
C vs B => 1 

Counting A-B and B-A as different is an easy query. But I can’t get them to be counted together.

Any suggestions?

Answer 1

SELECT Team1, Team2, SUM(num) FROM (
   SELECT Team1, Team2, COUNT(*) num
   FROM table_name
   GROUP BY Team1, Team2
  UNION ALL
   SELECT Team2, Team1, COUNT(*) num
   FROM table_name
   GROUP BY Team2, Team1
) combined
WHERE Team1 < Team2
GROUP BY Team1, Team2

Edit: Updated to reverse teams when necessary.

Note: This will run a lot faster than the versions using CASE you have been given in other answers because it will make full use of indexes.

Edit2: Moved the where to be even faster with indexes.