如何在SQL中实现类似行为？

Question

假设我有dogs(dogid)表和fleas(fleaid)表，另一个dogs_favorite_fleas(dogid,fleaid)狗可以在dog_favorite_fleas中列出他最喜欢的跳蚤。如何创建查询/临时表列出所有独特的跳蚤组并计算它们？例如，假设Rex喜欢flea a和flea b，而Spot喜欢'跳蚤and跳蚤c , and Lassie likes跳蚤b and跳蚤a`。查询结果应为：

fleas          |count
-------------------------
flea a, flea b |2
flea a, flea c |1

注意：有大约5000种不同的跳蚤和~450万只狗。

进行此查询的最佳方式是什么？

Answer 1

将GROUP_CONCAT与GROUP BY：

一起使用

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

SQL Fiddle Demo

更新：如果您要计算有多少组跳蚤列表，请将上一个查询括在子查询中，并将GROUP BY与COUNT一起使用，如下所示：

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

Updated SQL Fiddle Demo