Redshift SQL：使用日期和组考虑添加和重置计数器

Question

假设下面有一张桌子。我想要一个计数器来计算客户（有很多）在细分A中的次数。如果客户在两个季度之间跳到另一个细分，则当客户跳回到细分时计数器将重置答：我确信有很多方法可以做到这一点，但是我只是想不通。请帮助。谢谢！

Quarter    Segment    Customer    **Counter**
Q1 2018    A          A1          1
Q2 2018    A          A1          2
Q3 2018    A          A1          3
Q4 2018    B          A1          1
Q1 2019    B          A1          2
Q2 2019    A          A1          1
Q3 2019    A          A1          2

Answer 1

您可以将row_number()窗口函数与partition by子句一起使用来获得结果。

SELECT Quarter, Segment, Customer,
row_number() over (
PARTITION BY Quarter, Segment,Customer
ORDER BY Quarter
) as Counter
FROM <TABLE-NAME>

Answer 2

这是一种空白和岛屿问题。您可以使用不同的行号来解决此问题。真正的问题是处理宿舍。但是字符串函数可以处理。

select quarter, customer, segment,
       row_number() over (partition by customer, segment, seqnum - seqnum_cs order by right(quarter, 4), left(quarter, 2)) as counter
from (select t.*,
             row_number() over (partition by customer order by right(quarter, 4), left(quarter, 2)) as seqnum,
             row_number() over (partition by customer, segment order by right(quarter, 4), left(quarter, 2)) as seqnum_cs
      from t
     ) t
order by customer, seqnum;