An Issue for Num Seq.

Hi,

Without using number sequence functionality, how can I generate a sequence through coding ?

For example, Lets suppose I have 5 fields in my table, A,B,C,D,E. After creating the form of the table, when I shall click NEW button on the form, A must be initialised with +1, of previous stored value. For example, if the previous value stored is 8,  then on clicking NEW, it must be 8+1 = 9. How can I achieve this ?