Returrække med den maksimale værdi af én kolonne pr. gruppe

Dette er også muligt uden underforespørgsel:

SELECT DISTINCT
       id
      ,max(round) OVER (PARTITION BY id) AS round
      ,first_value(score) OVER (PARTITION BY id ORDER BY round DESC) AS score
FROM   SCORES
WHERE  id IN (1,2,3)
ORDER  BY id;

Returnerer præcis det, du bad om.
Det afgørende punkt er, at DISTINCT anvendes efter vinduesfunktioner.

SQL Fiddle.

Måske hurtigere, fordi den bruger det samme vindue to gange:

SELECT DISTINCT
       id
      ,first_value(round) OVER (PARTITION BY id ORDER BY round DESC) AS round
      ,first_value(score) OVER (PARTITION BY id ORDER BY round DESC) AS score
FROM   SCORES
WHERE  id IN (1,2,3)
ORDER  BY id;

Ellers gør du det samme.