売り手と買い手を一致させるSQLクエリ

@SérgioMichelsの答えは正しいはずです。しかし、それは売り手の名前を取得せず、（私の意見では）避けるべき構文を使用しています。

だから、ここに代替案があります...

SELECT
  buyer.name          AS buyer,
  buyerWants.name     AS buyer_wants,      (assuming the items have names),
  buyerHas.name       AS buyer_has,
  seller.name         AS seller,
  sellerWants.name    AS seller_wants,
  sellerHas.name      AS seller_has
FROM
  People              AS buyer
INNER JOIN
  Wishlist            AS buyerWants
    ON buyerWants.person_id = buyer.person_id
INNER JOIN
  Giveaways           AS sellerHas
    ON sellerHas.item_id = buyerwish.item_id
INNER JOIN
  People              AS seller
    ON seller.person_id = sellerHas.seller_id
INNER JOIN
  WishList            AS sellerWants
    ON sellerWants.person_id = seller.person_id
INNER JOIN
  GiveAways           AS buyerHas
    ON  buyerHas.item_id = sellerWants.item_id
    AND buyerHas.person_id = buyer.person_id
WHERE
  buyer.person_id = ?