Question

The database is quite simple. Below there is a part of a schema relevant to this question

• ROUND (round_id, round_number)

• TEAM (team_id, team_name)

• MATCH (match_id, match_date, round_id)

• OUTCOME (team_id, match_id, score)

I have a problem with query to retrieve data for all matches played. The simple query below gives of course two rows for every match played.

select * 
from round r 
inner join match m on m.round_id = r.round_id 
inner join outcome o on o.match_id = m.match_id 
inner join team t on t.team_id = o.team_id

How should I write a query to have the match data in one row?

Or maybe should I redesign the database – drop the OUTCOME table and modify the MATCH table to look like this:

• MATCH (match_id, match_date, team_away, team_home, score_away, score_home)?

Answer 1

You can almost generate the suggested change from the original tables using a self join on outcome table:

select o1.team_id team_id_1,
       o2.team_id team_id_2,
       o1.score score_1,
       o2.score score_2,
       o1.match_id match_id
from outcome o1
inner join outcome o2 on o1.match_id = o2.match_id and o1.team_id < o2.team_id

Of course, the information for home and away are not possible to generate, so your suggested alternative approach might be better after all. Also, take note of the condition o1.team_id < o2.team_id, which gets rid of the redundant symmetric match data (actually it gets rid of the same outcome row being joined with itself as well, which can be seen as the more important aspect).

In any case, using this select as part of your join, you can generate one row per match.