Question

我有一个SQL表（使用postgres），其中有两列：videoid，分类

videoid | classification
1       |20
1       |24
1       |24
1       |24
1       |24
2       |20
2       |20
2       |20
2       |20
2       |24
2       |24
3       |24
3       |24

我正在尝试检索最常见分类为24的所有videoid。（答案应仅为videoid 1和3）当我使用查询时：（从How to select most frequent value in a column per each id group?找到）

    SELECT DISTINCT ON (videoid) videoid, most_frequent_species FROM (
  SELECT videoid, classification AS most_frequent_species, count(*) as _count
FROM userviewed
GROUP BY videoid, classification) a
ORDER BY videoid, _count DESC
;

我检索结果集：

videoid     | most_frequent_species
    1       |24
    2       |20
    3       |24

但是当我尝试添加WHERE条款时：

WHERE classification = 24

我得到：

videoid| most_frequent_species
1      |24
2      |24
3      |24

如何创建只能检索的查询

videoid | most_frequent_species
1       |24
3       |24

Answer 1

您可以利用having子句（本质上是组后where子句）和mode函数：

select
    videoid
from
    userviewed
group by
    videoid
having
    mode() within group (order by classification) = 24

Answer 2

一种方法使用distinct on，然后进行过滤：

select *
from (select distinct on (videoid) videoid, category, count(*)
      from userviewed
      group by videoid, category
      order by videoid, count(*) desc
     ) vc
where category = 24;

SQL：仅选择第二列最常使用的值为某些值的ID（按ID分组）

2 个答案: