Question

So here’s a little background on the system. User’s battle and the winner is the one who wins the most rounds. I need help with possibly joining three tables. I have table user one, which stores the user info. Table match which stores match information. Table rounds which stores the winner of every rounds per match, so if a match has 5 rounds, then table rounds will have five rows for that match and will record the winner of each round.

Here’s some sample data:
Table user:
(userid is the primary key)

userid   username
-----------------
  1       Kevin
  2       Sam
  3       Steve
  4       Matt

Table match:
(id is the primary key. challenger and challenged are both foreign keys to user.userid)

 id  challenger  challenged  rounds
-----------------------------------
 1      2           3          3
 2      1           2          1
 3      2           3          3
 4      2           4          1

Table rounds:
(all fields are the primary key. id is foreign key to match.id and winner is foreign key to user.userid)

 id  round  winner
------------------
 1    1       2
 1    2       2
 1    3       3
 2    1       1
 3    1       2
 3    2       3
 3    3       2
 4    1       4

I’m trying to build a query that will output the following results:

winner  won   loser  won  
------------------------
 Sam     2    Steve   1
 Kevin   1    Sam     0
 Sam     2    Steve   1
 Matt    1    Sam     0

The above results shows the winner and loser of each of match. The won field shows the number of rounds won for that match for the winner and loser respectively.

Does anyone know how I can build the above query?

Answer 1

select 
    case when w1 > w2 then u1 else u2 end as winner,
    case when w1 > w2 then w1 else w2 end as won,
    case when w1 > w2 then u2 else u1 end as loser,
    case when w1 > w2 then w2 else w1 end as won
from (
    select m.id, u1.username as u1, u2.username as u2, 
        count(r1.winner) as w1,
        count(r2.winner) as w2
    from match m
    join user u1 on m.challenger = u1.userid
    join user u2 on m.challenged = u2.userid
    join rounds r on r.id = m.id
    left join rounds r1 on r.id = r1.id and r.round = r1.round and r1.winner = u1.userid
    left join rounds r2 on r.id = r2.id and r.round = r2.round and r2.winner = u2.userid
    group by m.id, u1.username, u2.username
) t

It relies on (Problems with NULL Values):

Aggregate (summary) functions such as COUNT(), MIN(), and SUM() ignore NULL values.

and here it is an advantage.