SQL Query X Days back excluding date ranges (Confusing!) - sql

Question

Ok, I have a tough SQL query, and I'm not sure how to go about writing it.
I am summing the number of "bananas collected" by an employee within the last X days, but what I could really use help on is determining X.
The "last X days" value is defined to be the last 100 days that the employee was NOT out due to Purple Fever, starting from some ChosenDate (we'll say today, 6/24/14). That is to say, if the person was sick with Purple Fever for 3 days, then I want to look back over the last 103 days from ChosenDate rather than the last 100 days. Any other reason the employee may have been out does not affect our calculation.
Table PersonOutIncident
+----------------------+----------+-------------+
| PersonOutIncidentID | PersonID | ReasonOut |
+----------------------+----------+-------------+
| 1 | Sarah | PurpleFever |
| 2 | Sarah | PaperCut |
| 3 | Jon | PurpleFever |
| 4 | Sarah | PurpleFever |
+----------------------+----------+-------------+
Table PersonOutDetail
+-------------------+----------------------+-----------+-----------+
| PersonOutDetailID | PersonOutIncidentID | BeginDate | EndDate |
+-------------------+----------------------+-----------+-----------+
| 1 | 1 | 1/1/2014 | 1/3/2014 |
| 2 | 1 | 1/7/2014 | 1/13/2014 |
| 3 | 2 | 2/1/2014 | 2/3/2014 |
| 4 | 3 | 1/15/2014 | 1/20/2014 |
| 5 | 4 | 5/1/2014 | 5/15/2014 |
+-------------------+----------------------+-----------+-----------+
The tables are established. Many PersonOutDetail records can be associated with one PersonOutIncident record and there may be multiple PersonOutIncident records for a single employee (That is to say, there could be two or three PersonOutIncident records with an identical ReasonOut column, because they represent a particular incident or event and the not-necessarily-continuous days lost due to that particular incident)
The nature of this requirement complicates things, even conceptually to me.
The best I can think of is to check for a BeginDate/EndDate pair within the 100 day base period, then determine the number of days from BeginDate to EndDate and add that to the base 100 days. But then I would have to check again that this new range doesn't overlap or contain additional BeginDate/EndDate pairs and add, if so, add those days as well. I can tell already that this isn't the method I want to use, but I can't wrap my mind quite around how exactly what I need to start/structure this query. Does anyone have an idea that might steer me in the correct direction? I realize this might not be clear and I apologize if I'm just confusing things.

Answer 1

One way to do this is to work with a table or WITH CLAUSE that contains a list of days. Let's say days is a table with one column that contains the last 200 days. (This means the query will break if the employee had more than 100 sick days in the last 200 days).
Now you can get a list of all working days of an employee like this (replace ? with the employee id):
WITH t1 AS
(
SELECT day,
ROW_NUMBER() OVER (ORDER BY day DESC) AS 'RowNumber'
FROM days d
WHERE NOT EXISTS (SELECT * FROM PersonOutDetail pd
INNER JOIN PersonOutIncidentID po ON po.PersonOutIncidentID = pd.PersonOutIncidentID
WHERE d.day BETWEEN pd.BeginDate AND pd.EndDate
AND po.ReasonOut = 'PurpleFever'
AND po.PersonID = ?)
)
SELECT * FROM t1
WHERE RowNumber <= 100;
Alternatively, you can obtain the '100th day' by replacing RowNumber <= 100 with RowNumber = 100.