BigQuery 窗口函数可匹配之前的记录

Question

我正在寻找一个窗口函数，通过比较当前记录中的值直到找到最近的匹配项来检索

BigQuery

中上一条记录中的值。这是一个示例表：

with a as (
  select 1 as hitNumber, 'xx' as sessionID, 'aaa' as pageName, null as convPageName
  union all
  select 2 as hitNumber, 'xx' as sessionID, 'ccc' as pageName, 'bbb' as convPageName
  union all
  select 3 as hitNumber, 'xx' as sessionID, 'ccc' as pageName, null as convPageName
  union all
  select 4 as hitNumber, 'xx' as sessionID, 'ddd' as pageName, 'qqq' as convPageName
  union all
  select 5 as hitNumber, 'xx' as sessionID, 'eee' as pageName, 'ccc' as convPageName
  
)
select *, ??? as prevConvPageName from a

基于此示例，查看

hitNumber = 5

，我想知道

convPageName

对应于最新记录，其中

convPageName

与

pageName

匹配，并且匹配记录上的

convPageName

不为空。在本例中，结果将为

bbb

，因为

convPageName

的

hitNumber = 5

与

pageName

的

hitNumber = 2

匹配。

Answer 1

这可以通过创建从最新的 pageName 到 convPageName 的映射表来解决。在下面的代码中，这被命名为

page_name_2_conv_page_name

。

请注意，我在表

a

中添加了额外的行，以证明使用了表

a

中基于 hitNumber 的最新记录。

WITH
  a AS (
  SELECT
    1 AS hitNumber,
    'xx' AS sessionID,
    'aaa' AS pageName,
    NULL AS convPageName
  UNION ALL
  SELECT 2,'xx','ccc','fff'
  UNION ALL
  SELECT 3,'xx','ccc','bbb'
  UNION ALL
  SELECT 4,'xx','ccc', null
  UNION ALL
  SELECT 5,'xx','ddd', 'qqq'
  UNION ALL
  SELECT 6,'xx','eee', 'ccc'
),
page_name_2_conv_page_name as (
select pageName,convPageName
from a
where convPageName is not null
QUALIFY row_number() over (partition by pageName order by hitNumber desc) = 1
)

select 
  a.*, 
  page_name_2_conv_page_name.convPageName
from 
  a left join page_name_2_conv_page_name 
    on a.convPageName = page_name_2_conv_page_name.pageName

结果