When you join two sheets, use 'left' join as the option. With left join, all records join between two sheets using 'account' show up + all non-matched records from sheet1 shows up with null as the amt2.
You can add a calc. field to sheet2 IFNULL(amt2,0). This will show 0 when there is null.
Hope this helps.
To replace "no data" with a 0 in Tableau, you can use the formula:
KK Molugu: his scenario deals with a data blend, not a join. Tableau handles these differently and I am not sure IFNULL will work in this case.
You are right.