SQL: How to fill empty cells with previous row value?

Question

I need to produce the column \"required\" in the following table using SQL without using loops and correlated sub queries. Is this possible in SQL 2008?

Date         


        

Answer 1

This is the "Last non-null puzzle," and here's one of several elegant solutions:

If your "sparse" table is SparseTable with columns Date, Customer, Value then:

with C as
(select *,
    max(case when Value is not null then [Date] end)
        over (partition by Customer order by [Date] rows unbounded preceding) as grp
 from SparseTable
)
insert into FullTable
select *, 
    max(Value) over (partition by Customer, grp order by [Date] rows unbounded preceding) as Required
from C

Where Value couldn't be filled forward it will still be NULL, so you can then

update FullTable set Required = 0 where Required is null