假设我有dogs(dogid)一张桌子和fleas(fleaid)一张桌子，而另一张桌子上dogs_favorite_fleas(dogid,fleaid)的狗可以将他最喜欢的跳蚤列在dog_favorite_fleas. 如何创建一个查询/临时表列出所有独特的跳蚤集并计算它们？例如，让我们说Rex喜欢flea a和flea b，并且Spot喜欢“跳蚤 aand跳蚤 c , andLassielikes跳蚤 band跳蚤 a”。查询的结果应该是：
fleas |count
-------------------------
flea a, flea b |2
flea a, flea c |1
注意：有约 5000 种不同的跳蚤和约 450 万只狗。
进行此查询的最佳方法是什么？
GROUP_CONCAT与 一起使用GROUP BY：
SELECT GROUP_CONCAT(f.flea SEPARATOR ', ') AS 'fleas', COUNT(df.dogid) 'Count'
FROM dogs_favorite_fleas df
INNER JOIN fleas f ON df.fleaid = f.id
INNER JOIN dogs d ON df.dogid = d.id
GROUP BY df.dogid
更新：如果您想计算有多少分组跳蚤列表，请将上一个查询包含在子查询中并使用GROUP BYwithCOUNT像这样：
SELECT fleas, COUNT(fleas) 'COUNT'
FROM
(
SELECT GROUP_CONCAT(f.flea SEPARATOR ', ') AS 'fleas'
FROM dogs_favorite_fleas df
INNER JOIN fleas f ON df.fleaid = f.id
INNER JOIN dogs d ON df.dogid = d.id
GROUP BY df.dogid
ORDER BY f.flea
) t
GROUP BY fleas