如何从Hive映射中获取唯一键列表

Question

我在Hive的一列中存储了一个映射，其中每行的键可以不同。如何从每个地图中获取仅键的列表？

Answer 1

[Function map_keys(Map)返回包含输入映射键的无序数组。

示例，请参见代码中的注释，展开数组并使用collect_set再次收集，它将返回不同键的数组：

    with mydata as (
    select 1 id, map('key11','val11','key12','val12','key13','val13') as mymap
    union all
    select 2 id, map('key21','val21','key22','val22','key13','val13') as mymap --Key13 also exist in first row
    )

select id, map_keys(d.mymap) keys
  from mydata d
; 

结果：

id  keys
1   ["key11","key12","key13"]
2   ["key21","key22","key13"]

如果您需要所有行的唯一键列表：

with mydata as (
select 1 id, map('key11','val11','key12','val12','key13','val13') as mymap
union all
select 2 id, map('key21','val21','key22','val22','key13','val13') as mymap --Key13 also exist in first row
)

select --id, 
       collect_set(key) as keys
  from mydata d
       lateral view outer explode(map_keys(d.mymap)) e as key
 --group by id   --without id in groupby you get the distinct list of keys in all rows
                 --with id in groupby you get list of map keys for each row
; 

结果：

["key11","key12","key13","key21","key22"]