Returns a unique integer value for each distinct evaluated value of expression encountered during the script execution. This function can be used e.g. for creating a compact memory representation of a complex key.
In order to create multiple counter instances if the autonumber function is used on different keys within the script, an optional parameter AutoID can be used for naming each counter.
Consider you have below data..
Month(Date) as Month
Customer, Date, Sales
A, 01/01/2015, 100
A, 10/02/2015, 120
A, 15/03/2015, 75
B, 15/01/2015, 120
B, 22/02/2015, 140
B, 18/03/2015, 190
C, 11/01/2015, 105
C, 20/02/2015, 120
C, 22/03/2015, 150
Now you want to associate Unique ID to each distinct customer... add below line
AutoNumber(Customer) as CustomerID,
If you want to associate Unique ID to each distinct customer per month... use as below
AutoNumber(Customer & Month,Customer) as CustomerMonthID;