3つのテーブルで子テーブルの数を数えたい場合のSQLうまくいきました
テーブル A{ aid } B{ bid, aid } C{ cid, aid } SELECT A.aid,COUNT(B.bid),COUNT(C.cid) FORM A LEFT JOIN B ON A.aid = B.aid LEFT JOIN C ON A.aid = C.aid GROUP BY A.aid
DISTINCTがCOUNT関数にいるのですね...
SELECT A.aid,COUNT(DISTINCT B.bid),COUNT(DISTINCT C.cid) FORM A LEFT JOIN B ON A.aid = B.aid LEFT JOIN C ON A.aid = C.aid GROUP BY A.aid