Mysql 按多个内部联接分组

我有一个数据集，可以表示为棒球卡交易应用程序。有两个表跟踪个人拥有的卡和个人想要的卡。如果一个人拥有一张以上特定玩家卡的副本，则“拥有的卡”表中将有多行，此处不包括球队。如果有个人想要添加到其收藏中的卡片，则会在“想要的卡片”表中插入一行

拥有的信用卡：

通缉证：

我试图写一份声明，将自动匹配个人寻求交易。如果一个人拥有一张以上的信用卡，则假定他们会交易一张额外的信用卡。同样，如果他们只持有一张玩家卡的副本，他们不会提供该卡进行交易。假设所有卡的价值相等

SELECT fred_wants.player AS 'Fred Wants', mary_has.player AS 'Mary Has'
FROM wanted_cards fred_wants
INNER JOIN owned_cards mary_has ON fred_wants.owner = mary_has.owner
INNER JOIN wanted_cards mary_wants ON mary_has.owner = mary_wants.owner
INNER JOIN owned_cards fred_has ON fred_has.owner = mary_wants.owner
WHERE fred_wants.owner = 'Fred'
GROUP BY mary_has.player
HAVING COUNT(mary_has.player) > 1

产生：

| Fred Wants | Mary Has   |
|------------|------------|
| Lou Gehrig | Babe Ruth  |
| Babe Ruth  | Lou Gehrig |
| Babe Ruth  | Yogi Berra |

期望输出：

| Fred Wants | Mary Has   |
|------------|------------|
| Babe Ruth  | Yogi Berra |

---

弗雷德想要一张卢·格里格的卡片，但玛丽只有一张，所以不能交易。通过去掉组，我明白了为什么返回行，但我不明白为什么会生成这么多行；我原以为HAVE会缩减它。

下面是一个示例查询，它返回谁有Fred想要的卡片的额外内容：

select w.player as 'Fred Wants', t.owner as 'Who has extras'
from
wanted_cards as w
inner join
(
  select owner, player
  from owned_cards as o
  where o.owner <> 'Fred'
  group by owner, player
  having count(*) > 1
) as t
on (w.player = t.player)
where w.owner = 'Fred'

---

主要的想法是选择弗雷德想要的所有牌，然后为所有其他玩家找到所有额外的牌

为什么弗雷德想要贝比·鲁斯！？！？此外，在进行更多操作之前，您还需要一个主键。@您已经太深入人心了，建议您重新阅读问题的第一句话。谢谢，我认为子查询实际上也更具可读性

| Fred Wants | Mary Has   |
|------------|------------|
| Babe Ruth  | Yogi Berra |

select w.player as 'Fred Wants', t.owner as 'Who has extras'
from
wanted_cards as w
inner join
(
  select owner, player
  from owned_cards as o
  where o.owner <> 'Fred'
  group by owner, player
  having count(*) > 1
) as t
on (w.player = t.player)
where w.owner = 'Fred'