title: DB design clarification for developing classified website
body: <p>I am about to start developing a classified web application. but I am facing certain difficulties 
  while building the DB design:</p>
  
  <p>User can post their advertisements under different types of pre-existing catagories (like Vehicles, Real Estate, Computers, Education etc). But each category have their certain specific fields/properties as well as some common fields/properties.</p>
  
  <p>My difficulties is like after filling a post Ad, say for Bike, I am saving the common i/p values to my advertisement_t table (as below).
  but where will I store other fields values specific to Bike like: Bike Manufacture, Bike Model, Bike Year etc?</p>
  
  <p>same way, if a user wanted to post an advertisement for House(rent), I can save the common i/p values to my advertisement_t table.
  but where will I store other fields values specific to House For Rent like: total area(sq), No of Rooms, etc. and how will I map.</p>
  
  <p>My Tables are below:</p>
  
  <pre><code>category_id
  ------------
  (catId, parentCatId, cat_title, cat_desc)
  
  advertisement_t
  ----------------
  (adId, catId, userId, ad_title, ad_desc, photoId, postDate, statusId, price, ad_address, adValidFrom, adValidTo, adIsDeleted) 
  
  Photo_t
  --------
  (photoId, primary_photo, sec_photo1, sec_photo_2, sec_photo_3)
  
  user_t
  -------
  (userId, username, password, email, activationkey, isvalidated, joinedDate, activatedDate, statusId)
  </code></pre>
  
  <p>I dont have any idea how other classified sites(like olx, freeadds) used to maintain(persist &amp; display) their posted advertisement data on their database.</p>
  
  <p>If any one has any idea, please help.</p>
  
  <p>Thanks in advance.</p>