sql - automatically select and check database for existing records -

i need , suggestions

i have

2 combo boxes

1 cmbcategory 2nd cmbsubcategory

and 1 textbox equipment_id

the process when chose computer on category combobox , system_unit on sub category combobox

it data inputted in 2 combo boxes

resulting to

equipment_id textbox = computer-system_unit

and last is. check id number in database if exist, if exist increment , add +1 on id number

so if there computer-system_unit-1 on database

the result on textbox of equipment_id "computer-system_unit-2"

any suggestions, comments , answer appreciated