Creating multiple row per row based on date caclulation - sql

Question

I have never done anything like it and not able to find any guidance when I google. How can I create multiple row based on one row data
For example here's my data:
Employee | EmploymentDate
1 1/1/2017
Every three months I need to calculate level.
Employee | EmploymentDate | Level | LevelDateRange
1 1/1/2017 1 1/1/2017 - 3/31/2017
1 1/1/2017 2 4/1/2017 - 6/30/2017
1 1/1/2017 3 7/1/2017 - 9/30/2017
LevelDateRange Calculation:
Start Date: If 1st level then EmploymentDate
else previous LevelDateRange end date + 1
End Date: If 1st level then EmployemeentDate + 3 months minus a day
else Start date + 3 months
Any suggestion?

Answer 1

In your case, you might consider cross apply:
select d.employee, v.*
from mydata d cross apply
(values (1, employmentdate, dateadd(day, -1, dateadd(month, 3, d.employmentdate))),
(2, dateadd(month, 3, employmentdate), dateadd(day, -1, dateadd(month, 6, d.employmentdate))),
(3, dateadd(month, 6, employmentdate), dateadd(day, -1, dateadd(month, 9, d.employmentdate)))
) v(lev, startdate, enddate);
I would advise you to keep the start date and end date in separate columns. Combine them into a string at the application level or when you query the database.

Answer 2

A tally table and a CTE:
DECLARE #StartDate DATE
SELECT #StartDate = EmploymentDate FROM emp
DECLARE #MonthsSinceStart INT = DATEDIFF(mm,#startDate,GETDATE())
DECLARE #NumLevels INT = #MonthsSinceStart / 3
--100 row tally table
IF OBJECT_ID('tempdb..#Tally') IS NOT NULL DROP TABLE #Tally
SELECT ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) AS Level
INTO #Tally
FROM (VALUES(0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) a(n)
CROSS JOIN (VALUES(0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) b(n);
WITH cte AS
(
SELECT Employee,
EmploymentDate,
Level,
EmploymentDate AS StartDate,
DATEADD(dd,-1,DATEADD(mm,3,EmploymentDate)) AS EndDate
FROM emp
CROSS JOIN #Tally
WHERE #Tally.Level =1
UNION ALL
SELECT Employee,
EmploymentDate,
Level + 1,
DATEADD(dd,1,EndDate) AS StartDate,
DATEADD(dd,-1,DATEADD(mm,3,DATEADD(dd,1,EndDate))) AS EndDate
FROM cte
WHERE cte.Level < #NumLevels
)
SELECT Employee,
EmploymentDate,
Level,
CONVERT(NVARCHAR(10),StartDate) + ' - ' + CONVERT(NVARCHAR(10),EndDate)
FROM cte;
Suggest you give it a test with more than one row in your table