DateDiff In months between two different types date formats - sql

Question

I have two date formats, EndDate in YYYYMMDD format and another Monthfirstdate in YYYY-MM-DD FORMAT. I need to get the difference between these two dates.
SET NOCOUNT ON
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
DECLARE #Date DATE = dbo.value('DATE')
SELECT
E.EmployeeID
,E.StartDateKey
,E.EndDateKey
,E.FromDateKey
,E.ToDateKey
INTO #Employee
FROM
dbo.Employee E
---clean--
DECLARE #StartDate DATE = (SELECT DATEADD(YY, DATEDIFF(YY, 0, #Date) - 1, 0))
DECLARE #EndDate DATE = DATEADD(S, -1, DATEADD(MM, DATEDIFF(MM, 0, #Date) + 3, 0))
SELECT
E.EmployeeID
,CAST(CONVERT(VARCHAR(10), E.StartDateKey, 112) AS INT) AS _EmployeeStartDateKey
,CAST(CONVERT(VARCHAR(10), E.EndDateKey, 112) AS INT) AS _EmployeeFinishDateKey
,CAST(CONVERT(VARCHAR, D.MonthFirstDay, 112) AS INT) AS _DateKey
,???? AS [Tenure]
FROM #Employee AS E
INNER JOIN dbo.Date D ON
D.DateKey BETWEEN E.FromDateKey AND E.ToDateKey
WHERE
D.DateKey BETWEEN #tartDate AND #EndDate
AND D.DayOfMonth = 1

Answer 1

You can try below query in SQL Server:
DECLARE
#firstDate DATETIME = '2022-12-01',
#dateinStringFormat VARCHAR(20)= '20121201',
#endDate DATETIME;
--Converting date string to Datetime format(YYYY-MM-DD)
SELECT #endDate = CONVERT(CHAR(10), CONVERT(datetime, #dateinStringFormat), 120);
--Now, Calculating the date difference in Month
SELECT DATEDIFF(MONTH, #endDate, #firstDate) AS DateDiff;