Count Total Experience of a employee in sql query

Then take case statement and check if the Emp in employee previous exprience details table , if he not there then show current exp as a total exp.

For ex:
EmpPresentCompDtls- EmpID,EmpName, JoiningDate,LeavingDate
EmpPrevoisCompDtls-EmpID,PreviousComp, JoiningDate,LeavingDate


select EmpId,EmpName,(case when e1.EmpId =e2.EmpId then (SELECT DATEDIFF(YY,e2.JoiningDate,e2.LeavingDate)+DATEDIFF(YY,e1.JoiningDate,e1.LeavingDate)) else (SELECT DATEDIFF(YY,e1.JoiningDate,e1.LeavingDate)) end) as TotalExp from EmpPresentCompDtls e1,EmpPrevoisCompDtls e2 where e1.EmpId=e2.EmpId


May be Sytax errors check once, but the logic is correct 

I agree with you but this is not my query.....
Let me explain all the query...

I have a table that contain employee information such as EmpID,EmpName, JoiningDate, etc....

I have one another table that contain employee previous exprience details such as Emp_previous_Company,Joining_Date,Leaving_Date,Last_CTC etc...

I have a grid view like that....

-----------------------------------------------------------------------------------------------------
EmpID     EmpName        Desig          Deptt        DOJ                 Currt_Exp      Total_Exp
-----------------------------------------------------------------------------------------------------
101         Ramesh           Developer      IT          13 Jan 2013    01 Yr 6 M        Blank
101         Sureh              Developer      IT          01 Nov 2012    01 Yr 8 M        5 Yr         
102         Mohan            Programmer    IT          01 Nov 2011    02 Yr 8 M       Blank 
103         Sohan             Web Design   IT          01 Nov 2010    03 Yr 8 M        5 Yr 
104         Mukesh             Network       IT          01 Nov 2009    04 Yr 8 M        5 Yr 
-----------------------------------------------------------------------------------------------------

If Total_Exp coloum is Blank than it mean that employee have not any previous companies work experience.

My question is if Total_Exp column is Blank than total_Exp value is current_exp....... 

How to write a query.......
Plz plz solve it.......

SELECT DATEDIFF(DAY,JoiningDate,LeavingDate) then it will return in Days

SELECT DATEDIFF(YY,JoiningDate,LeavingDate) then it will return in Years