SELECT C.NIF, C.nombre, COUNT(P.cod_prueba)
FROM   Candidato C, Prueba P
WHERE  C.cod_cand = P.cod_cand
GROUP BY C.nombre, C.NIF

UNION

SELECT C1.NIF, C1.nombre, 0
FROM   Candidato C1
WHERE  C1.cod_cand NOT IN (SELECT P.cod_cand FROM Prueba P2);