SELECT T1.PK, T1.C, T1.D, T2.CNT
FROM A T1
LEFT OUTER JOIN  (SELECT FK, COUNT(1) AS CNT FROM B GROUP BY FK)  T2
ON T1.PK = T2.FK

-- 그룹핑 후 조인 --
SELECT a.pk, a.c, a.d
     , IFNULL(b.cnt, 0) cnt
  FROM a a
  LEFT OUTER JOIN (SELECT fk, COUNT(*) cnt FROM b GROUP BY fk) b
    ON a.pk = b.fk
;

-- 조인 후 그룹핑 --
SELECT a.pk, a.c, a.d
     , COUNT(b.fk) cnt
  FROM a a
  LEFT OUTER JOIN b b
    ON a.pk = b.fk
 GROUP BY a.pk, a.c, a.d
;