Building a HR Dataset in T-SQL, would like a row start date column and a row end date column

Question

I have HR dataset problem. For a position, I have position details such as category, type, etc. These details change over time and independently of each other, resulting in the following datasets:

Position    StartDate      EndDate
1           '2020-01-01'   '3000-01-01'

Position    Type        StartDate     EndDate
1           'Part-time' '2020-01-01'  '2020-07-31'
1           'Full-time' '2020-08-01'  '2020-08-30'
1           'Part-time' '2020-09-01'  '3000-01-01

Position    Category      StartDate     EndDate
1           'Agency'      '2020-01-01'  '2020-06-30'
1           'Employee'    '2020-07-01'  '2020-08-30'
1           'Contractor'  '2020-09-01'  '3000-01-01

So from this, I would like to get a dataset such as:

Position   Type         Category      StartDate      EndDate
1          'Part-time'  'Agency'      '2020-01-01'   '2020-06-30'
1          'Part-time'  'Employee'    '2020-07-01'   '2020-07-31'
1          'Full-time'  'Employee'    '2020-08-01'   '2020-08-30'
1          'Part-time'  'Contractor'  '2020-09-01'   '3000-01-01'

So that when filtered on a date, it will bring back the situation for that position on that date. For this example on '2020-07-05', the position will show as a part-time employee.

Methods I've looked at so far include CASE statements for creating the StartDate and EndDate fields

And creating a union of the datasets, and running ROW_NUMBER() OVER(PARTITION BY Position OVER BY StartDate, EndDate) to try to filter out the wrong rows when these tables are joined.

Edit: Apologies, I should have added, this is the minimum example of the problem. The example I'm looking at will have 6-12 columns. So the solution will need to be scalable.

Answer 1

The method I'd choose is to apply a CROSS JOIN between the Types and the Categories table with filtering by overlapping phases of both tables.

SELECT * FROM Position p
INNER JOIN Type t
    ON t.Position = p.Position
CROSS JOIN Category c
WHERE c.StartDate >= t.StartDate AND c.StartDate <= t.EndDate
    OR (t.StartDate >= c.StartDate AND t.StartDate <= c.EndDate)
    OR (c.StartDate < t.StartDate AND c.EndDate > t.EndDate)
ORDER BY t.StartDate

In the SELECT list you can apply your CASE Statement to choose the right date for your date range.