SQL Server - 如果行是组中的第一条记录，则更新列

Question

如何确定每个品牌和类别具有价值的第一行，然后将“FirstValue”列更新为 1，否则更新为 0？ 例如预期的表格

Date   |  Brands  |  Category |  Value  | FirstValue 
Jan 08 |   A      |  1        | 0       |0
Jan 08 |   A      |  2        | 0       |0
Jan 08 |   A      |  3        | 0       |0
Jan 08 |   B      |  1        | 12      |1
Jan 08 |   B      |  2        | 0       |0
Jan 08 |   B      |  3        | 0       |0
Feb 08 |   A      |  1        | 5       |1
Feb 08 |   A      |  2        | 0       |0
Feb 08 |   A      |  3        | 67      |1
Feb 08 |   B      |  1        | 0       |0
Feb 08 |   B      |  2        | 0       |0
Feb 08 |   B      |  3        | 6       |1

Answer 1

Common table expressions 实际上是可更新的，将 CTE 与排名函数 ROW_NUMBER() 相结合提供了一个完美的解决方案：

with cte as (
 select row_number() over (partition by Brand, Category order by Date) as rn
, FirstValue
from Table)
update cte
set FirstValue = case when rn = 1 then 1 else 0 end;