How does DATEADD work when joining the same table with itself? - sql

Question

I have a table with monthly production values.
Example:
Outdate | Prod Value | ID
2/28/19 | 110 | 4180
3/31/19 | 100 | 4180
4/30/19 | 90 | 4180
I also have a table that has monthly forecast values.
Example:
Forecast End Date | Forecast Value | ID
2/28/19 | 120 | 4180
3/31/19 | 105 | 4180
4/30/19 | 80 | 4180
I want to create a table that has a row that contains the ID, the Prod Value, the current month (example: March) forecast, the previous month forecast, the next month forecast.
What I want:
ID | Prod Value | Outdate | Current Forecast | Previous Forecast | Next Forecast
4180 | 100 | 3/31/19 | 105 | 120 | 80
The problem is that when I used DATEADD to bring in the specific value from the Forecast table for the previous month, random months are missing from my final values.
I've tried adding in another LEFT JOIN / INNER JOIN with the DateDimension table when adding in the Next Month and Previous Month forecast, but that either does not solve the problem or adds in too many rows.
My DateDimension table that has these columns: DateKey
Date, Day, DaySuffix, Weekday, WeekDayName, IsWeekend, IsHoliday, DOWInMonth, DayOfYear, WeekOfMonth, WeekOfYear, ISOWeekOfYear, Month, MonthName, Quarter, QuarterName, Year, MMYYYY, MonthYear, FirstDayOfMonth, LastDayOfMonth, FirstDayOfQuarter, LastDayOfQuarter, FirstDayOfYear, LastDayOfYear, FirstDayOfNextMonth, FirstDayOfNextYear
My query is along these lines (abbreviated for simplicity)
SELECT A.ArchiveKey, BH.ID, d.[Date], BH.Outdate, BH.ProdValue, BH.Forecast, BHP.Forecast, BHN.Foreceast
FROM dbo.BudgetHistory bh
INNER JOIN dbo.DateDimension d ON bh.outdate = d.lastdayofmonth
INNER JOIN dbo.Archive a ON bh.ArchiveKey = a.ArchiveKey
LEFT JOIN dbo.BudgetHistory bhp ON bh.ID = bhp.ID AND bhp.outdate = DATEADD(m, - 1, bh.Outdate)
LEFT JOIN dbo.BudgetHistory bhn ON bh.ID = bhn.ID AND bhn.outdate = DATEADD(m, 1, bh.Outdate)
WHERE bh.ID IS NOT NULL
I get something like this:
+------+------------+---------+------------------+-------------------+---------------+
| ID | Prod Value | Outdate | Current Forecast | Previous Forecast | Next Forecast |
+------+------------+---------+------------------+-------------------+---------------+
| 4180 | 110 | 2/28/19 | 120 | NULL | NULL |
| 4180 | 100 | 3/31/19 | 105 | 120 | 80 |
| 4180 | 90 | 4/30/19 | 80 | NULL | NULL |
+------+------------+---------+------------------+-------------------+---------------+
And the pattern doesn't seem to follow anything reasonable.
I want the values to be filled in for each row.

Answer 1

You could join the tables, then use window functions LEAD() and LAG() to recover the next and previous forecast values:
SELECT
p.ID,
p.ProdValue,
p.Outdate,
f.ForecastValue,
LAG(f.ForecastValue) OVER(PARTITION BY f.ID ORDER BY f.ForecastEndDate) PreviousForecast,
LEAD(f.ForecastValue) OVER(PARTITION BY f.ID ORDER BY f.ForecastEndDate) NextForecast
FROM prod p
INNER JOIN forecast f ON p.ID = f.ID AND p.Outdate = f.ForecastEndDate
This demo on DB Fiddle with your sample data returns:
ID | ProdValue | Outdate | ForecastValue | PreviousForecast | NextForecast
---: | --------: | :------------------ | ------------: | ---------------: | -----------:
4180 | 110 | 28/02/2019 00:00:00 | 120 | null | 105
4180 | 100 | 31/03/2019 00:00:00 | 105 | 120 | 80
4180 | 90 | 30/04/2019 00:00:00 | 80 | 105 | null

Answer 2

DATEADD only does end of month adjustments if the newly calculated value isn't a valid date. So DATEADD(month,-1,'20190331') produces 28th February. But DATEADD(month,-1,'20190228') produces 28th January, not the 31st.
I would probably go with GMB's answer. If you want to do something DATEADD based though, you can use:
bhp.outdate = DATEADD(month, DATEDIFF(month,'20010131', bh.Outdate) ,'20001231')
This always works out the last day of the previous month from bh.OutDate, but it does it by computing it as an offset from a fixed date, and then applying that offset to a different fixed date.
You can just reverse the places of 20010131 and 20001231 to compute the month after rather than the month before. There's no significance about them other than them both having 31 days and having the "one month apart" relationship we're wishing to apply.