SQL Server IF 0, add value for following date - sql

Question

I am writing this query in MSSQL Server Management Studio, and I have run into a mental road block. Let's say I have a few buildings and as they have come online I want to compare their production vs what they were forecasted, P_AMOUNT and F_AMOUNT. These are two separate databases combined into one with a query (The production db and forecast db that is).
So my problem here is. I want to select the peak production here along side the forecast each B_ID. Most of the buildings are like B_ID #2 and have a forecast amount, but occasionally there is one that does not. Like B_ID #1. How would I go about rolling the date one month forward for only the F_Date if the F_Amount = 0?
SELECT P1.B_ID, P1.P_DATE, P1.P_AMOUNT, F1.F_DATE, F1.F_AMOUNT
FROM DB1.Production P1
INNER JOIN DB2.Forecast F1
ON P1.DB_ID = F1.DB_ID

Answer 1

Use a nested select of the Forecast table so that you get the minimum forecast date after the production date that has a value, something like:
SELECT MIN(F1.F_DATE)
WHERE F1.F_DATE > P1.P_DATE
AND F1.F_AMOUNT > 0

Answer 2

There are a handful of ways to solve your problem. One way is to rank each row based on F_DATE and when the F_AMOUNT value is zero, to pull the next row. Since you did not specify a version of SQL Server, I used syntax that would work in SQL Server 2005 or later.
With RnkItems As
(
Select B_ID, P_DATE, P_AMOUNT, F_DATE, F_AMOUNT
, Row_Number() Over ( Order By F_DATE ) As Rnk
From SourceData
)
Select R.B_ID, R.P_DATE, R.P_AMOUNT
, R.F_DATE As [Original F_DATE]
, R.F_AMOUNT As [Original F_AMOUNT]
, Case R.F_AMOUNT
When 0 Then R1.F_DATE
Else R.F_DATE
End As F_DATE
, Case R.F_AMOUNT
When 0 Then R1.F_AMOUNT
Else R.F_AMOUNT
End As F_AMOUNT
From RnkItems As R
Left Join RnkItems As R1
On R1.Rnk = R.Rnk + 1
SQL Fiddle Version
If you are using SQL Server 2012, you can use the new Lead function:
Select B_ID, P_DATE, P_AMOUNT
, F_DATE As [Original F_DATE]
, F_AMOUNT As [Original F_AMOUNT]
, Case F_AMOUNT
When 0 Then Lead ( F_DATE, 1 ) Over ( Order By F_DATE )
Else F_DATE
End As F_DATE
, Case F_AMOUNT
When 0 Then Lead ( F_AMOUNT, 1 ) Over ( Order By F_DATE )
Else F_AMOUNT
End As F_AMOUNT
From SourceData
SQL Fiddle Version
The above two solutions rely on the stated requirement that you always go exactly one month ahead. If, however, you want to go the first month with a non-zero value (i.e., it could be multiple jumps), that's different:
Select S.B_ID, S.P_DATE, S.P_AMOUNT
, S.F_DATE As [Original F_DATE]
, S.F_AMOUNT As [Original F_AMOUNT]
, Case S.F_AMOUNT
When 0 Then (
Select Min( S2.F_DATE )
From SourceData As S2
Where S2.F_DATE >= S.F_DATE
And S2.F_AMOUNT <> 0
)
Else S.F_DATE
End As F_DATE
, Case S.F_AMOUNT
When 0 Then (
Select S1.F_AMOUNT
From SourceData As S1
Where S1.F_DATE = (
Select Min( S2.F_DATE )
From SourceData As S2
Where S2.F_DATE > S.F_DATE
And S2.F_AMOUNT <> 0
)
)
Else S.F_AMOUNT
End As F_AMOUNT
From SourceData As S
SQL Fiddle Version