How to choose multiple values with dropdown lists from SQL database?

also i am stuck with another problem.
i am trying to make a search page where people can search based on the city and the category using dropdown menus…

i have already created a database of city , category and merchant

so when the user selects the city and the category from dropdown menu list of merchant appears of that particular city and category.

just like this website www.customerdeal.com.au when you go to search you see what i mean (by the way i know this website sucks but its just the same search thing i am trying to do in my website).

regards,
:tie: