避免多次加入同一个表的更好方法

Question

我有一张桌子上的动物，其结构如下所示

AnimalId    Feature Present
--------    ------- -------
1           Teeth   Yes
1           Leg     Yes
2           Teeth   No
2           Leg     Yes 
3           Teeth   Yes
3           Leg     Yes

如果牙齿和腿都是'是'，我需要检索动物 我写了一个像

这样的查询

select distinct A1.AnimalId from Animal A1
inner join Animal A2 on
A1.AnimalId = 
(select distinct A2.AnimalId from Animal A2 
 inner join Animal A3 on
 A2.AnimalId = 
(select distinct A3.AnimalId from Animal A3 where A3.Feature = 'Leg' and A3.Present = 'Yes'  group by A3.AnimalId)
where A2.Feature = 'Teeth' and A2.Present = 'Yes'  group by A2.AnimalId)

及其工作。

想知道有没有更好的方法来写这个并获得相同的结果。

Answer 1

我喜欢使用group by和having来处理此类查询。在你的情况下：

select a.animalId
from animal a
where (a.feature = 'Teeth' and a.present = 'Yes') or
      (a.feature = 'Leg' and a.present = 'Yes')
group by a.animalId
having count(distinct a.feature) = 2;

where子句可以简化为：

where a.feature in ('Teeth', 'Leg') and a.present = 'Yes'