使用 group by 和 count (postgres) 計算行

Question

嗨，我在 PostgreSQL 中查詢時遇到問題。 這是我的表imgs_tags示例數據：

img_id | tag_id
   1        2
   1        3
   2        2
   2        3
   3        2
   3        3

我試過：

select count(img_id) from imgs_tags where tag_id IN(2,3) group by img_id having count(tag_id) = 2

它返回如下：

count:
  2
  2 
  2

但我希望它返回count = 3 。 我該如何解決？ 謝謝

Answer 1

您可以像這樣使用子查詢

select count(*) from (select count(img_id) from imgs_tags where tag_id IN(2,3) group by img_id having count(tag_id) = 2) as temp_img_tags;

returns 3

Answer 2

你應該這樣寫：

select count(*) from (
select img_id
from imgs_tags 
where tag_id IN (2,3) 
group by img_id
having count(tag_id) = 2
) t

或者如果你想對這些數據做一些進一步的操作，那么更好地使用Common Table Expression (with clause)如下所示：

with cte as (
select img_id
from imgs_tags 
where tag_id IN (2,3) 
group by img_id
having count(tag_id) = 2
) 

select count(*) from cte