Combining rows with overlapping dates in T-SQL - sql-server

Question

I have some data similar to the below:
Base data
Student Start Date End Date Course
John 01-Jan-20 30-Sep-20 Business
John 01-Jan-20 30-Dec-20 Psychology
John 01-Oct-20 NULL Music
Jack 01-Feb-20 30-Sep-20 Business
Jack 01-Apr-20 30-Nov-20 Music
I want to transform the data so I have a row for each student, for each time period, with a concatenated list of courses, i.e.
Target output
Student Start Date End Date Course
John 01-Jan-20 30-Sep-20 Business, Psychology
John 01-Oct-20 30-Dec-20 Psychology, Music
John 01-Jan-21 NULL Music
Jack 01-Feb-20 31-Mar-20 Business
Jack 01-Apr-20 30-Sep-20 Business, Music
Jack 01-Oct-20 30-Nov-20 Music
I have a script that works if the dates are identical, using STUFF on the course field and grouping on student/dates (code below). But I can't work out how to handle the overlapping dates?
Select Student,
Courses =
STUFF((select ',' + course
from Table1 b
where a.student = b.student
for XML PATH('')
),1,1,''
)
from table1 a
Group by student

Answer 1

This is a little long winded, as you need to get the groups for the dates. As the dates don't overlap, you then need to do a bit of elimination of some of the groupings too, so it takes a couple of sweeps.
I use CTEs to get the groups I need, and then use a subquery to string aggregate (on a more recent version of SQL Server you can use STRING_AGG and not need a second scan of the table). This ends up with this:
WITH YourTable AS(
SELECT *
FROM (VALUES('John',CONVERT(date,'01-Jan-20'),CONVERT(date,'30-Sep-20'),'Business'),
('John',CONVERT(date,'01-Jan-20'),CONVERT(date,'30-Dec-20'),'Psychology'),
('John',CONVERT(date,'01-Oct-20'),CONVERT(date,NULL),'Music'),
('Jack',CONVERT(date,'01-Feb-20'),CONVERT(date,'30-Sep-20'),'Business'),
('Jack',CONVERT(date,'01-Apr-20'),CONVERT(date,'30-Nov-20'),'Music'))V(Student,StartDate,EndDate,Course)),
Dates AS(
SELECT DISTINCT V.Student, V.[Date]
FROM YourTable YT
CROSS APPLY (VALUES(YT.Student,YT.StartDate),
(YT.Student,YT.EndDate)) V(Student,[Date])),
Islands AS(
SELECT *,
LEAD(ISNULL([Date],'99991231')) OVER (PARTITION BY Student ORDER BY ISNULL([Date],'99991231')) AS NextDate
FROM Dates
WHERE [Date] IS NOT NULL),
Groups AS(
SELECT I.Student,
I.Date AS StartDate,
CASE DATEPART(DAY,I.NextDate) WHEN 1 THEN DATEADD(DAY, -1, I.NextDate) ELSE I.NextDate END AS EndDate,
STUFF((SELECT ',' + YT.Course
FROM YourTable YT
WHERE YT.Student = I.Student
AND YT.StartDate <= I.[Date]
AND (YT.EndDate >= I.NextDate OR YT.EndDate IS NULL)
ORDER BY YT.Course
FOR XML PATH(''),TYPE).value('(./text())[1]','nvarchar(MAX)'),1,1,'') AS Courses
FROM Islands I)
SELECT Student,
StartDate,
EndDate,
Courses
FROM Groups
WHERE ([StartDate] != EndDate OR EndDate IS NULL)
AND Courses IS NOT NULL
ORDER BY Student DESC,
StartDate ASC;