Question

我正在使用Java和SQL，而我正试图计算宠物园里的所有山羊。我知道我可以使用以下查询：

  SELECT COUNT(1) FROM PettingZoo pz WHERE pz.animalType = 'GOAT'

足够简单。现在，让我们说山羊有三种颜色：棕色，黑色或灰色。如果我想计算每种颜色的数量，我可以执行以下查询：

  SELECT COUNT(1) FROM PettingZoo pz WHERE pz.animalType = 'GOAT' AND
       pz.animalType.color = 'BROWN'

  SELECT COUNT(1) FROM PettingZoo pz WHERE pz.animalType = 'GOAT' AND
       pz.animalType.color = 'BLACK'

  SELECT COUNT(1) FROM PettingZoo pz WHERE pz.animalType = 'GOAT' AND
       pz.animalType.color = 'GREY'

此方法有效，但速度很慢。这样做会打开表四次：一次获得总计数，另外三次打开各个颜色。 有没有办法在一个查询中返回所有四种颜色？

编辑：为清楚起见，我们假设有15只山羊。九个是棕色，0个是黑色，6个是灰色。第一个查询将返回15.第二个查询将返回9.第三个和最后一个查询将分别返回0和6。我正在寻找一个一次返回所有四个值的东西（所以我不会在四个不同时间读同一个表）。可能是阵列？ {15,9,0,6}

Answer 1

  SELECT SUM(case when pz.animalType.color = 'BROWN' then 1 else 0 end) as browns,
         SUM(case when pz.animalType.color = 'BLACK' then 1 else 0 end) as blacks,
         SUM(case when pz.animalType.color = 'GREY'  then 1 else 0 end) as greys ,
         COUNT(1) as total
  FROM PettingZoo pz WHERE pz.animalType = 'GOAT' AND
         (
           pz.animalType.color = 'BROWN' or 
           pz.animalType.color = 'BLACK' or 
           pz.animalType.color = 'GREY'
         )
  WHERE pz.animalType is not null

Answer 2

您可以使用group by获取一个查询中的所有计数：

SELECT 
      pz.animalType.color, COUNT(*)
FROM PettingZoo pz
WHERE pz.animalType = 'GOAT'
GROUP BY pz.animalType.color

Answer 3

我认为Kevin忘记了组子句：

SELECT pz.animalType.color, COUNT(*) FROM PettingZoo pz WHERE
pz.animalType = 'GOAT' GROUP BY pz.animalType.color;

你能在一个SQL语句中返回多个COUNT吗？

3 个答案: