Cela a fonctionné pour moi avec la correction de la jointure gauche.
SELECT mdate, team1,
SUM(CASE WHEN teamid = team1 THEN 1 ELSE 0 END) score1,
team2,
SUM(CASE WHEN teamid = team2 THEN 1 ELSE 0 END) score2
FROM game LEFT JOIN goal ON matchid = id GROUP BY mdate, matchid, team1, team2
