In worksheet #1
Each row contains a unique name like: David, Joe etc. (in column A) and

it's data like: salary etc. (in columns B,C etc.). Last column contains

unique groupName like group1, group2 etc.

In worksheet #2
Each row contains a unique group name like: group1, group2 etc. (in
column A) and calculated data like: groupAverageSalary etc. (in columns

B,C etc.)

In worksheet#2 I calaculate group's data dynamic according to
user input in worksheet#1 (For example: groupAverageSalary for group1
will be calculated as the average salary of all the people who are
members in group1

I used:
=SUMIF(worksheet#1!E1:E15,worksheet#2!A1,worksheet #1!B1:B15)/COUNTIF(worksheet#1!E1:E15,worksheet#2!A1)

My problem:
In case that there no values for some group in worksheet#1 column B
I want to display NULL in worksheet#2 column B (my formula returns 0).

How can I changed my formula to do this ?

worksheet#1
A,B,... ,E
Name, salary, ..., groupName
David, 2000$, ..., group1
Joe, 1000$, ..., group2


worksheet#2
A,B, ...
groupName, averageSalary, ...
group1, 2000$, ...
group2, 1000$