Comparing SQL date ranges across multiple nullable date columns - sql-server

Question

I have a calendar type check I'm trying to do on SQL Server. For each month of the year, I need to check if the employee was hired or not. There can be an original hire date, a rehire date, a termination date, and the last termination date; other than the original hire date, which will always have a value, all of these date fields can be null.
Given the following data:
EmpID OrigHireDate TermDate LastTermDate RehireDate
42 2017-09-25 NULL 2019-03-26 2019-10-30
What I am trying to achieve is the following result for each month for last year (i.e. 2019) and having no luck in coming up with the right statement. Assume I already have a table containing each month's number along with the start/end date of the month that I can use to compare the date ranges.
EmpID Month EmployeeDuring
42 1 True
42 2 True
42 3 True
42 4 False
42 5 False
42 6 False
42 7 False
42 8 False
42 9 False
42 10 True
42 11 True
42 12 True

Answer 1

The following works. May need some minor adjustments to handle all possible combinations of EmpID, OrigHireDate, TermDate, LastTermDate, RehireDate
I apologize for posting mostly code. Will add more explanation and or comments tomorrow.
DECLARE #EmpID int, #OrigHireDate date, #TermDate date, #LastTermDate date, #RehireDate date
DECLARE #year int
SET #year = 2019
SET #EmpID = 42
SET #OrigHireDate = '2017-09-25'
SET #TermDate = NULL
SET #LastTermDate = '2019-03-26'
SET #RehireDate = '2019-10-30'
SET #OrigHireDate = DATEADD(day,-DAY(#OrigHireDate)+1, #OrigHireDate)
SET #LastTermDate = DATEADD(day,-DAY(ISNULL(#LastTermDate,GETDATE()))+1, #LastTermDate)
SET #RehireDate = DATEADD(day,-DAY(#RehireDate)+1, #RehireDate)
SET #TermDate = DATEADD(day,-DAY(ISNULL(#TermDate,GETDATE()))+1, #TermDate)
;WITH CTE_DATES_ORIGINAL([Date],[Level])
AS
(
SELECT #OrigHireDate AS [DATE],
1 AS [Level]
UNION ALL
SELECT
DATEADD(MONTH,1, [DATE] ) , [Level] + 1
FROM CTE_DATES_ORIGINAL
WHERE [DATE] < ISNULL(#LastTermDate,GETDATE())
),
CTE_DATES_REHIRE([Date],[Level])
AS
(
SELECT #RehireDate AS [DATE],
1 AS [Level]
UNION ALL
SELECT
DATEADD(MONTH,1, [DATE] ) , [Level] + 1
FROM CTE_DATES_REHIRE
WHERE [DATE] < ISNULL(#TermDate,GETDATE())
),
CTE_DATES_YEAR(m) AS
(
SELECT 1
UNION ALL
SELECT m+1
FROM CTE_DATES_YEAR
WHERE m < 12
)
SELECT #EmpID AS EmpID, m AS [Month], ISNULL(EmployeeDuring.EmployeeDuring,0) AS EmployeeDuring
FROM CTE_DATES_YEAR y
LEFT OUTER JOIN
(
SELECT
[Date], 1 AS EmployeeDuring
FROM
CTE_DATES_ORIGINAL
UNION
SELECT
[Date] , 1 AS EmployeeDuring
FROM
CTE_DATES_REHIRE
) employeeDuring
ON DATEADD(month,m-1, CAST(CAST(#year AS CHAR(4)) + '-1-1' AS DATE)) = employeeDuring.[Date]
ORDER BY m
OPTION (MAXRECURSION 5000)