すべての共著者を検索-多対多のマッピングテーブルのファセット/グループ化

これを試してください：

SELECT "AuthorId", COUNT(*)
FROM BookAuthorMapping
WHERE "BookId" IN (SELECT "BookId" FROM BookAuthorMapping WHERE "AuthorId" = 1)
GROUP BY "AuthorId"

デモはこちら

または、INNER JOINを使用することもできます ：

SELECT t1."AuthorId", COUNT(*)
FROM BookAuthorMapping AS t1
INNER JOIN BookAuthorMapping AS t2 ON t1."BookId" = t2."BookId" AND t2."AuthorId" = 1
GROUP BY t1."AuthorId"

デモはこちら