Hello! I am trying to add rows based on data in a particular column. For example, I have the following:

A 2 X
A 3 X
A 5 X
B 2 X
B 4 X
B 5 X
C 1 X
C 5 X

I need to make sure, even if there is not an assciated value for the column C (shown by having a value of X above) that I know something does not exist. For example:

A 1 Null
A 2 X
A 3 X
A 4 Null
A 5 X
B 1 Null
B 2 X
B 3 Null
B 4 X
B 5 X
C 1 X
C 2 Null
C 3 Null
C 4 Null
C 5 X

Thank you for your help!