Question

假设我有下表，我如何按ID分组，并获得每列中最常见的值附： table很大，我需要为很多列做这个

ID  Col1  Col2  Col3....
1   A     null
1   A     X
1   B     null
1   A     Y
2   C     X
2   C     Y
2   A     Y
3   B     Z
3   A     Z
3   A     Z
3   B     X
3   B     Y

预期结果：

ID  Col1  Col2  Col3....
1   A     null
2   C     Y
3   B     Z

Answer 1

这是一种方法，使用分析函数和keep：

select id,
       min(col1) keep(dense_rank first order by cnt_col1 desc) as col1_mode,
       min(col2) keep(dense_rank first order by cnt_col2 desc) as col2_mode,
       min(col3) keep(dense_rank first order by cnt_col3 desc) as col3_mode
from (select id,
             count(*) over (partition by id, col1) as cnt_col1,
             count(*) over (partition by id, col2) as cnt_col2,
             count(*) over (partition by id, col3) as cnt_col3
      from t
     ) t
group by id;

最常见的值在统计中称为“模式”，Oracle提供了计算此功能的函数。因此，更简单的方法是使用stats_mode()：

   select id,
         stats_mode(col1) as mode_col1,
         stats_mode(col2) as mode_col2,
         stats_mode(col3) as mode_col3
  from table t
  group by id;

编辑：

如评论中所述，stats_mode()不会计算NULL个值。解决此问题的最简单方法是找到一些不在数据中的值并执行：

   select id,
          stats_mode(coalesce(col1, '<null>')) as mode_col1,
          stats_mode(coalesce(col2, '<null>')) as mode_col2,
          stats_mode(coalesce(col3, '<null>')) as mode_col3
  from table t
  group by id;

另一种方法是恢复第一种方法或类似方法：

select id,
       (case when sum(case when col1 = mode_col1 then 1 else 0 end) >= sum(case when col1 is null then 1 else 0 end)
             then mode_col1
             else NULL
        end) as mode_col1,
       (case when sum(case when col2 = mode_col2 then 1 else 0 end) >= sum(case when col2 is null then 1 else 0 end)
             then mode_col2
             else NULL
        end) as mode_col2,
       (case when sum(case when col3 = mode_col13 then 1 else 0 end) >= sum(case when col3 is null then 1 else 0 end)
             then mode_col3
             else NULL
        end) as mode_col3
from (select t.*,
             stats_mode(col1) over (partition by id) as mode_col1,
             stats_mode(col2) over (partition by id) as mode_col2,
             stats_mode(col3) over (partition by id) as mode_col3
      from table t
     ) t
group by id;

Oracle如何在多列的列中获得最常见的值？

1 个答案: