how to select max of mixed string/int column?
HKL9 (string) is greater than HKL15, because they are compared as strings. One way to deal with your problem is to define a column function that returns only the numeric part of the invoice number.
If all your invoice numbers start with HKL, then you can use:
SELECT MAX(CAST(SUBSTRING(invoice_number, 4, length(invoice_number)-3) AS UNSIGNED)) FROM tableIt takes the invoice_number excluding the 3 first characters, converts to int, and selects max from it.