根据行的过去值添加计算字段 SQL

Question

我有一个包含日期字段和列 'X' 的数据集，如下所示。 X 是一个二进制字段 - 我想在末尾添加一个字段 ('desired field (calculated)')，它将为我提供字段 'X' 的字符串 1 开始的第一个日期。有任何想法吗？在 Excel 中，我会根据以下内容查看之前的行以检查它是否为空白：

逻辑：

Case when X=0 then Null 
     when X=1 and 'desired field value for the row above' = Null, then Date
     when X=1 and 'desired field value for the row above' != Null, then value of 'desired field value for the row above'
) end as desired field

Date (dd/mm/yy)|X|desired field (calculated)
01/01/00       | |
02/01/00       |1|02/01/00
04/01/00       |1|02/01/00
10/01/00       |1|02/01/00
20/01/00       | |
20/02/00       | |
20/03/00       | |
04/04/00       |1|04/04/00
06/04/00       |1|04/04/00
10/04/00       |1|04/04/00
20/05/00       | |

Answer 1

您可以使用累加和为 1 分配一个分组，然后散布值：

select t.* except (grp),
       (case when x = 1 then max(date) over (partition by grp) end) as desired
from (select t.*,
             countif(x <> 1) over (order by date) as grp
      from t
     ) t;

Answer 2

考虑以下方法

select * except(new_grp, grp),
  case 
    when x = 1 then first_value(date) over(partition by grp order by cast(date as date format 'DD/MM/YY'))
    else null
  end desired_field
from (
  select *, countif(new_grp) over(order by cast(date as date format 'DD/MM/YY')) grp
  from (
    select *, x != lag(x) over(order by cast(date as date format 'DD/MM/YY')) new_grp
    from `project.dataset.table`
  )
)
# order by cast(date as date format 'DD/MM/YY')

如果应用于示例数据 - 输出为

根据行的过去值添加计算字段 SQL

Add calculated field based on past values of the row SQL

sql

google-bigquery

case

common-table-expression