get the value from the previous row if row is NULL - sql

Question

I have this pivoted table
+---------+----------+----------+-----+----------+
| Date | Product1 | Product2 | ... | ProductN |
+---------+----------+----------+-----+----------+
| 7/1/15 | 5 | 2 | ... | 7 |
| 8/1/15 | 7 | 1 | ... | 9 |
| 9/1/15 | NULL | 7 | ... | NULL |
| 10/1/15 | 8 | NULL | ... | NULL |
| 11/1/15 | NULL | NULL | ... | NULL |
+---------+----------+----------+-----+----------+
I wanted to fill in the NULL column with the values above them. So, the output should be something like this.
+---------+----------+----------+-----+----------+
| Date | Product1 | Product2 | ... | ProductN |
+---------+----------+----------+-----+----------+
| 7/1/15 | 5 | 2 | ... | 7 |
| 8/1/15 | 7 | 1 | ... | 9 |
| 9/1/15 | 7 | 7 | ... | 9 |
| 10/1/15 | 8 | 7 | ... | 9 |
| 11/1/15 | 8 | 7 | ... | 9 |
+---------+----------+----------+-----+----------+
I've found this article that might help me but this only manipulate one column. How do I apply this to all my column or how can I achieve such result since my columns are dynamic.
Any help would be much appreciated. Thanks!

Answer 1

The ANSI standard has the IGNORE NULLS option on LAG(). This is exactly what you want. Alas, SQL Server has not (yet?) implemented this feature.
So, you can do this in several ways. One is using multiple outer applys. Another uses correlated subqueries:
select p.date,
(case when p.product1 is not null else p.product1
else (select top 1 p2.product1 from pivoted p2 where p2.date < p.date order by p2.date desc)
end) as product1,
(case when p.product1 is not null else p.product1
else (select top 1 p2.product1 from pivoted p2 where p2.date < p.date order by p2.date desc)
end) as product1,
(case when p.product2 is not null else p.product2
else (select top 1 p2.product2 from pivoted p2 where p2.date < p.date order by p2.date desc)
end) as product2,
. . .
from pivoted p ;
I would recommend an index on date for this query.

Answer 2

I would like to suggest you a solution. If you have a table which consists of merely two columns my solution will work perfectly.
+---------+----------+
| Date | Product |
+---------+----------+
| 7/1/15 | 5 |
| 8/1/15 | 7 |
| 9/1/15 | NULL |
| 10/1/15 | 8 |
| 11/1/15 | NULL |
+---------+----------+
select x.[Date],
case
when x.[Product] is null
then min(c.[Product])
else
x.[Product]
end as Product
from
(
-- this subquery evaluates a minimum distance to the rows where Product column contains a value
select [Date],
[Product],
min(case when delta >= 0 then delta else null end) delta_min,
max(case when delta < 0 then delta else null end) delta_max
from
(
-- this subquery maps Product table to itself and evaluates the difference between the dates
select p.[Date],
p.[Product],
DATEDIFF(dd, p.[Date], pnn.[Date]) delta
from #products p
cross join (select * from #products where [Product] is not null) pnn
) x
group by [Date], [Product]
) x
left join #products c on x.[Date] =
case
when abs(delta_min) < abs(delta_max) then DATEADD(dd, -delta_min, c.[Date])
else DATEADD(dd, -delta_max, c.[Date])
end
group by x.[Date], x.[Product]
order by x.[Date]
In this query I mapped the table to itself rows which contain values by CROSS JOIN statement. Then I calculated differences between dates in order to pick the closest ones and thereafter fill empty cells with values.
Result:
+---------+----------+
| Date | Product |
+---------+----------+
| 7/1/15 | 5 |
| 8/1/15 | 7 |
| 9/1/15 | 7 |
| 10/1/15 | 8 |
| 11/1/15 | 8 |
+---------+----------+
Actually, the suggested query doesn't choose the previous value. Instead of this, it selects the closest value. In other words, my code can be used for a number of different purposes.

Answer 3

First You need to add identity column in temporary or hard table then resolved by following method.
--- Solution ----
Create Table #Test (ID Int Identity (1,1),[Date] Date , Product_1 INT )
Insert Into #Test ([Date], Product_1)
Values
('7/1/15',5)
,('8/1/15',7)
,('9/1/15',Null)
,('10/1/15',8)
,('11/1/15',Null)
Select ID , DATE ,
IIF ( Product_1 is null ,
(Select Product_1 from #TEST
Where ID = (Select Top 1 a.ID From #TEST a where a.Product_1 is not null and a.ID<b.ID
Order By a.ID desc)
),Product_1) Product_1
from #Test b
-- Solution End ---