You should split up your table into two tables.
One table containing the accounts, one table containing account transactions.
Apart from this, the reason why you want to add an extra column is not valid.
You can use this basic query though for now:
SELECT Amount
FROM YourTable, YourTable AS YourTable_1
WHERE YourTable.account=YourTable_1.account
AND YourTable.sbu=YourTable_1.sbu
AND YourTable.agn=YourTable_1.agn;
being YourTable the name of your database table.
RV