如何 select 属于一组但不属于另一组的项目

Question

Sqlite3

我如何 select 只是宠物而不是食物的动物？ POF 是“宠物或食物”栏。动物可以同时属于这两个组。这是真实问题的缩小版。我不想把它分成更多的表。

animal  pof
----------
fish    pet
fish    food
pig     food
cat     pet
dog     pet
horse   pet
mouse   pet
duck    pet
duck    food
cow     food
rabbit  pet
rabbit  food
gerbil  pet
worm    <null>
chicken food

我有以下的，但看起来很别扭：

SELECT * from 
(SELECT  NAME, POF, count(*) as cnt
 FROM    ANIMALS
 GROUP BY NAME) AS GC
 WHERE GC.cnt == 1 AND GC.POF == 'pet'

正确屈服：

NAME    POF cnt
---------------
cat     pet  1
dog     pet  1
gerbil  pet  1
horse   pet  1
mouse   pet  1

Answer 1

一种方法使用聚合：

select animal, count(*) cnt
from animals
group by animal
having min(pof) = max(pof) an min(pof) = 'pet'

如果没有重复项，如您的数据所示，计数始终为 1...您可以使用 not exists 产生相同的结果（取决于您的数据，这可能会或可能不会更有效率）：

select animal
from animals a
where 
    pof = 'pet' 
    and not exists (select 1 from animals a1 where a1.animal = a.animal and a1.pof <> 'pet')

Answer 2

使用NOT IN排除所有具有pof = 'food':

的动物

select *
from animals
where pof = 'pet'
and animal not in (select animal from animals where pof = 'food')

或者如果您只需要 animal 列，您可以使用 EXCEPT:

select animal from animals where pof = 'pet'
except
select animal from animals where pof = 'food'

参见demo。

Answer 3

您可以使用聚合的另一种方式

select animal
from animals
group by animal
having avg(case when pof='pet' then 1 else 0 end)=1;

如何 select 属于一组但不属于另一组的项目

How to select items which belong to one group but not another

sql

sqlite

count

subquery

relational-division