SQL query to split records by intervals - sql-server

Question

Let's assume I have a table which has columns From and To which are dates and a bit type column which identifies whether it is a cancel (1 = cancel). Also an Id which is a PK and CancelId which references what is cancelled.
Let's say I have records which look like:
Id From To IsCancel CancelId
1 2015-01-01 2015-01-31 0 NULL
2 2015-01-03 2015-01-09 1 1
3 2015-01-27 2015-01-31 1 1
I am expecting the result to show what intervals of then non-cancel records are still uncancelled:
Id From To
1 2015-01-01 2015-01-02
1 2015-01-10 2015-01-26
I can make it so it would split each record into dates, then subtract cancelled dates from the records then merge the intervals but since I have quite a lot of records, I find this very inefficient and am pretty sure that I am overlooking something simple.

Answer 1

The task you want to achieve is non trivial. A possible solution involves placing all From / To dates in an ordered sequence. The following UNPIVOT operation:
SELECT ID, EventDate, StartStop,
ROW_NUMBER() OVER (ORDER BY ID, EventDate, StartStop) AS EventRowNum,
IsCancel
FROM
(SELECT ID, IsCancel, [From], [To]
FROM Event) Src
UNPIVOT (
EventDate FOR StartStop IN ([From], [To])
) AS Unpvt
produces this result set:
ID EventDate StartStop EventRowNum IsCancel
--------------------------------------------------
1 2015-01-01 From 1 0
2 2015-01-03 From 2 1
2 2015-01-09 To 3 1
3 2015-01-27 From 4 1
3 2015-01-31 To 5 1
1 2015-01-31 To 6 0
Using a CTE, you can subsequently simulate LEAD function (available from SQL Server 2012 onwards) in order to place in a single record the current and the next date from the sequence above:
;WITH StretchEventDates AS
(
-- above query goes here
), CTE AS
(
SELECT s.ID, s.EventDate, s.StartStop, s.IsCancel,
sLead.EventDate As LeadEventDate, sLead.StartStop AS LeadStartStop, sLead.IsCancel AS LeadIsCancel
FROM StretchEventDates AS s
LEFT JOIN StretchEventDates AS sLead ON s.EventRowNum + 1 = sLead.EventRowNum
)
The above produces the following result set:
ID EventDate StartStop IsCancel LeadEventDate LeadStartStop LeadIsCancel
--------------------------------------------------------------------------------------
1 2015-01-01 From 0 2015-01-03 From 1
2 2015-01-03 From 1 2015-01-09 To 1
2 2015-01-09 To 1 2015-01-27 From 1
3 2015-01-27 From 1 2015-01-31 To 1
3 2015-01-31 To 1 2015-01-31 To 0
1 2015-01-31 To 0 NULL NULL NULL
Using CASE statements you can filter these records in order to get the desired output.
Putting it all together:
;WITH StretchEventDates AS
(
SELECT ID, EventDate, StartStop,
ROW_NUMBER() OVER (ORDER BY EventDate, StartStop) AS EventRowNum,
IsCancel
FROM
(SELECT ID, IsCancel, [From], [To]
FROM Event) Src
UNPIVOT (
EventDate FOR StartStop IN ([From], [To])
) AS Unpvt
), CTE AS
(
SELECT s.ID, s.EventDate, s.StartStop, s.IsCancel,
sLead.EventDate As LeadEventDate, sLead.StartStop AS LeadStartStop, sLead.IsCancel AS LeadIsCancel
FROM StretchEventDates AS s
LEFT JOIN StretchEventDates AS sLead ON s.EventRowNum + 1 = sLead.EventRowNum
), CTE_FINAL AS
(SELECT *,
CASE WHEN StartStop = 'From' AND IsCancel = 0 THEN EventDate
WHEN StartStop = 'To' AND IsCancel = 1 THEN DATEADD(d, 1, EventDate)
END AS [From],
CASE WHEN LeadStartStop = 'From' AND LeadIsCancel = 1 THEN DATEADD(d, -1, LeadEventDate)
WHEN LeadStartStop = 'To' AND LeadIsCancel = 0 THEN LeadEventDate
END AS [To]
FROM CTE
)
SELECT ID, [From], [To]
FROM CTE_FINAL
WHERE [From] IS NOT NULL AND [To] IS NOT NULL AND [From] <= [To]
You may have to add additional CASEs in the query above to handle additional combinations of 'cancelations' following 'non-canceled' (and vice-versa) events.
With the data provided in the OP the above yields the following output:
ID From To
---------------------------
1 2015-01-01 2015-01-02
2 2015-01-10 2015-01-26