Another way to improve the SQL Query to avoid union?

Question

User can search by Postcode (eg: L14, L15, L16) or Location from a textbox.

If user type in \"Liverpool\", it will find all the shops that are located in \"Liverpool

Answer 1

Since all tables and selected columns are the same, you can simply do this:

  SELECT DISTINCT shops.*, DA.delivery_cost, DA.postcode AS AreaPostcode FROM shops
             JOIN shops_delivery_area as DA on DA.shop_id = shops.id
   WHERE (DA.postcode = "Liverpool")
      OR (DA.postcode = shops.postcode AND shops.location = "Liverpool")

Like you said in Diego's answer, the conditions are a litle different! So, you compensate that difference in the WHERE clause.