Auto Fill A Field With The Latest Release In The Database

I have the following table:

id

number

currentbalance

previous balance
For example if you want to automatically fill in the number field, I use the maximum ID value +1: as below

{

$ rs = CustomQuery ("select max (". AddFieldWrappers ("Number"). ") from the max_number". AddTableWrappers ("caixa"));

db_fetch_array $ data = ($ rs);

if ($ data) {

$ values ​​['number'] = $ data ['max_number'] + 1;

}
How would I auto populate the field current_balance with the last release. Because I am not using the maximum and minimum value for the field value is variable.
Sorry for my English

João