How to match specific elements that user enters with database? -php/mysql/ajax

Question

I have a database of users. Each user has to enter during registration a street address, state, city, zipcode.

I also have in my database tables for States, Cities, Counties, Zipcodes. These tables are filled with all the US states, counties, cities and zipcodes available.

-Zipcodes are linked to cities only. 
-Cities are linked to counties and states. Each city has unique city_id
-Counties are linked to states only. Each county has unique county_id
-Each state has unique state_id

My PHP page specifically for addresses consists of:

-SELECT for States (state_name as text; state_id as value)
-TEXTBOX for City (with only city_name as value)
-TEXTBOX for Zipcode

I use PHP Session to store values entered by user, in case of failed form validation.

When the user submits I want to be able to put in the database the city_id (since that id will also contain county_id and state_id). But what the user submits is a city_name. How can I match that?

Do i need a search in the database on the form validation and see if I can find the city based on what the user entered? I suppose I would need a perfect match.

Use autocomplete and retrieve with jquery and ajax cities (including the city_id) based on what the user types and then store somewhere the city_id value?

How should I do it?

Answer 1

Your query to get city_id based on either city_name or zipcode might look like this

SELECT c.city_id 
  FROM cities c JOIN zipcodes z
    ON c.city_id = z.city_id
 WHERE c.city_name = 'Baltimore' 
    OR z.zipcode = '21201'
 GROUP BY c.city_id

UPDATE To get city_id by one of its zipcodes

SELECT city_id 
  FROM zipcodes
 WHERE zipcode = '21201'
 GROUP BY city_id

Here is SQLFiddle demo