Combine multiple rows of same column into one with a comma separation

Hi Friends,

I have one requirement where

1) Students can  have one staff or multiple staff

Database values :

Class Code      Staff Name

A                       Steve

A                       David

if count(staff)=1 then  

staff1

else 

staff1 & "," & staff2 & "," & staff 3

I have tried to use Join & Lookup but ended up with same staff value is adding like

staff1 & "," & staff1  

Please help me in solving this issue.

Thanks,

Sam