Question

所以我正在研究一个SQL问题，我有一个酒吧和啤酒数据库，我想从中：列出所有供应迈克喜欢的啤酒的酒吧，以及不止一个饮酒者经常光顾的啤酒。

数据库包括：

likes (drinker, beer)
frequents(dinker, bar)
sells(bar, beer)

所以我试过了：

SELECT bar
FROM beer.sells
WHERE beer IN 
(SELECT beer
FROM beer.likes
WHERE drinker = 'Mike') AND bar IN
(SELECT bar FROM beer.frequents HAVING COUNT(drinker) > 1)

哪个不起作用...为什么计数（饮酒者）比较不能作为过滤器过滤出频繁出现频率少于2个饮酒者的酒吧？

Answer 1

在条形子查询中看起来需要GROUP BY bar。否则，您将从COUNT(*)有效地获得beer.frequents，这不是您想要的：

SELECT bar
FROM beer.sells
WHERE beer IN 
(SELECT beer
FROM beer.likes
WHERE drinker = 'Mike') AND bar IN
(SELECT bar FROM beer.frequents GROUP BY bar HAVING COUNT(drinker) >= 2)

Answer 2

您需要'GROUP BY'才能使聚合的COUNT有效：

SELECT bar
 FROM beer.sells
 WHERE beer IN 
  (SELECT beer
    FROM beer.likes
    WHERE drinker = 'Mike'
  )
 AND bar IN
  (SELECT bar
    FROM beer.frequents
    GROUP BY bar
    HAVING COUNT(drinker) > 1
   )

Answer 3

“经常出现超过一个饮酒者。”＆lt;＆gt; HAVING COUNT(drinker) > 2（ 1 ）

SELECT bar
FROM beer.sells
WHERE beer IN 
(SELECT beer
FROM beer.likes
WHERE drinker = 'Mike') 
AND bar IN
(SELECT bar 
 FROM beer.frequents 
 GROUP BY bar
 HAVING COUNT(drinker) > 1)

如何按成员数过滤SQL元组

3 个答案: