Arithmetic Expressions And Data Displaying

Hello, Im new to SQL. Im currently having a table called Daily with 5 columns. TesterNames,Activity,Hours_given,Hours_used and Delta. I have grabbed the data for the first three colums from another table called Tester. For Hours_used I supposed to get the data from another server after i get the access but mean time i just put my own data so that i can check the value of Delta. Data for Delta column should be as below.

Delta = Hours_given - Hours_used

So How do I do the codings for this expression and display it in the same table. Output I need as below:

TesterNames        Activity       Hours_given       Hours_used          Delta

abc                       A                    5                       6                     -1

def                        B                    7                       6                      1

 

I have used

INSERT INTO Daily ( TesterNames,Activity,Hours_given)

SELECT ( Tester_ID,Weekday_Day,EntityWDD)

FROM Tester

and the first 3 columns are filled with the data needed. But when I use

INSERT INTO DAILY(Delta) 

SELECT Delta = Hours_given - Hours_used FROM DAILY

The output was like below

TesterNames        Activity       Hours_given       Hours_used          Delta

abc                       A                    5                       6                    

def                        B                    7                       6                     

Null                     Null                  Null                   Null                    -1

Null                     Null                  Null                   Null                     1

Help me . thank you.