Average value depending on cumulative value from other column aka running total - sql-server

Question

I've sifted through the various sql-server tagged threads using AVERAGE and Cumulative as search terms. Various desperate answers, but I can't cobble them together for my needs. The use case is to find the initial average value (cumulative value/cumulative days on) for a time period when cumulative days on is greater than 60 and less than 90.
Below is a table where ID identifies the object, VALUE is the amount reported on a monthly basis and DAYSON is the number of days in that month where the object ran to produce the value. YEARMONTH is date value on which on can sort.
ID VALUE DASYON YEARMONTH
1 166 27 201502
1 1 2 201505
1 569 19 201507
1 312 19 201508
2 364 27 201502
2 328 31 201503
2 242 29 201504
2 273 31 201505
2 174 30 201506
2 188 25 201507
2 203 25 201508
3 474 28 201502
3 521 31 201503
3 465 30 201504
3 473 31 201505
3 434 30 201506
3 404 31 201507
I would like to create a summary table that averages the cumulative value divided by the cumulative days uniquely for each ID where cumulative days is greater than 60 and less than 90. Below is a table that with the cumulative values. (I generated this in Excel)
ID VALUE cumValue DASYON cumDaysOn YEARMONTH
1 166 166 27 27 201502
1 1 167 2 29 201505
1 569 736 19 48 201507
1 312 1048 19 67 201508
2 364 364 27 27 201502
2 328 692 31 58 201503
2 242 934 29 87 201504
2 273 1207 31 118 201505
2 174 1381 30 148 201506
2 188 1569 25 173 201507
2 203 1772 25 198 201508
3 474 474 28 28 201502
3 521 505 31 59 201503
3 465 535 30 89 201504
3 473 566 31 120 201505
3 434 596 30 150 201506
3 404 627 31 181 201507
I try this based on other threads:
SELECT
ID,
Value,
SUM(Value) OVER (ORDER BY ID, YearMonth) [cumValue],
DaysOn,
SUM (DaysOn) OVER (Order by ID, YearMonth) as cumDaysOn,
YearMonth
FROM table
WHERE DAYSON > 0 and Liquid > 0 and YearMonth > 201501
GROUP BY ID, YearMonth, Value, DaysOn
ORDER BY ID, yearmonth
I can't get it to iterate over the ID; it just keeps summing down the column. If I could create a table or view like the one above, then I could always use a select statement and divide cumvalue by cumdayson.
Below is a table to show where I would get the initial average value (InititalAverageValue) based on the criteria:
ID VALUE cumValue DASYON cumDaysOn YEARMONTH InitalAvgValue
1 166 166 27 27 201502
1 1 167 2 29 201505
1 569 736 19 48 201507
1 312 1048 19 67 201508 55
2 364 364 27 27 201502
2 328 692 31 58 201503
2 242 934 29 87 201504 32
2 273 1207 31 118 201505
2 174 1381 30 148 201506
2 188 1569 25 173 201507
2 203 1772 25 198 201508
3 474 474 28 28 201502
3 521 505 31 59 201503
3 465 535 30 89 201504 18
3 473 566 31 120 201505
3 434 596 30 150 201506
3 404 627 31 181 201507
Ultimately what I desire is table as such:
ID InitalAvgValue
1 55
2 32
3 18
Thanks in advance for any help.

Answer 1

The crux is that you need a running total. There are several approaches to calculating running totals, but they have various tradeoffs between simplicity and performance. The "best" approach depends on the expected size of your data set and whether you are using SQL Server 2012 or an earlier version. The following article describes some different options along with the pros and cons:
http://sqlperformance.com/2012/07/t-sql-queries/running-totals
Here's a quick example using correlated subqueries, which may be reasonable for small data sets, but likely would not scale well to larger data:
SELECT
ID,
ROUND(AVG(CAST(CumulativeValue AS FLOAT) / CAST(CumulativeDaysOn AS FLOAT)), 1) AS Average
FROM
(
SELECT
ID,
Value,
DaysOn,
(SELECT SUM(Value) FROM ExampleTable t2 WHERE t1.ID = t2.ID and t2.YearMonth <= t1.YearMonth) AS CumulativeValue,
(SELECT SUM(DaysOn) FROM ExampleTable t2 WHERE t1.ID = t2.ID and t2.YearMonth <= t1.YearMonth) AS CumulativeDaysOn
FROM
ExampleTable t1
) AS ExampleWithTotals
WHERE
CumulativeDaysOn > 60 AND CumulativeDaysOn < 90
GROUP BY
ID
ORDER BY
ID
;
Output:
ID Average
1 15.6
2 10.7
3 16.4