1) In case you want to display values of zipcode in dropdown, you could simply create a dropdown and map the value with zipcode column, it will automatically pickup. Or you could define dropdown values.
2) Or, have simple validation test whether zipcode is empty and zipcode value is equal to 6 ( In case of India). For data validation, you could design a technical workflow which will delete( or necessary action) data.
You can create a jssp pages in dynamic jssp pages where in you can accept the query string for the zip code and then according to that query string you can search for the zip code in the the adobe schema.
From your landing page, As soon as a customer enter the zipcode you can send an Ajax request to the jssp page and get the response and display on the landing page.