how to get the number on months between two dates in sql server 2005

Question

I have a column in my sql server 2005 table that should hold the number of months an employee has been in service.

Since I also have the date the employee was engaged, I want the "months_In_Service" column to be a computed column.

Now if I use DATEDIFF(month,[DateEngaged],GETDATE()) as the formula for the months in service computed column, the results are correct some times and other times incorrect.

What would be the better reliable way to get the number of months between the DateEngaged value and the current date? Which formula should i use in my computed column?

Solution 1

Something like (might need to swap the 1 and 0, untested)

datediff(month,[DateEngaged],getdate()) +
 CASE WHEN DATEPART(day, [DateEngaged]) < DATEPART(day, getdate()) THEN 1 ELSE 0 END

DATEDIFF measure month boundaries eg 00:00 time on 1st of each month, not day-of-month anniversaries

Edit: after seeing OP's comment, you have to subtract 1 if the start day > end day

DATEDIFF (month, DateEngaged, getdate()) -
 CASE
   WHEN DATEPART(day, DateEngaged) > DATEPART(day, getdate()) THEN 1 ELSE 0
 END

So for 20 Dec to 13 Jan, DATEDIFF gives 1 and then 20 > 13 so subtract 1 = zero months.

Solution 2

Same approach as gbn, but with less keystrokes :-)

SELECT 
    DATEDIFF(MONTH, DateEngaged, GETDATE()) +
    CASE 
        WHEN DAY(DateEngaged) < DAY(GETDATE())
        THEN 1 
        ELSE 0 
    END

Solution 3

Maybe you want something like:

(year(getdate())-year([DateEngaged]))*12+(month(getdate())-month([DateEngaged]))