Alta solutie , putin mai complexa comparativ cu cea oferita de Razvan (se pleaca de la ideea ca exista in setul de inregistrari o cheie primara/candidat):

Tabela are structura T(id cheie primara, a, b, c) 

SELECT a, b
FROM T
WHERE id in (SELECT MAX(id) AS id FROM T GROUP BY a,b)
ORDER BY b,c