I have a static prompt which is a single select. In that I have two values lets call it A and B. So when I select option 'A' my report pulls all data from the DB which is expected. So when user Select option 'B' the report should pull only the records whose code = 'M'. Here code is a column name in the report.
Note: For option 'A' I don't need to set any prompt in the report because it should pull all records by default.
Let's assume your parameter name is param and data item is named item.
Filter expression:
if (?param? = 'A')
then ([item])
else ('M')
= [item]
Note: You absolutely need to use a prompt. The result of selecting A should be to not filter.