Question

I am building a database similar to the one described here where I have products of different type, each type with its own attributes.

I report a short version for convenience

product_type
============
product_type_id INT
product_type_name VARCHAR

product
=======
product_id INT
product_name VARCHAR
product_type_id INT -> Foreign key to product_type.product_type_id
... (common attributes to all product) 

magazine
========
magazine_id INT
title VARCHAR
product_id INT -> Foreign key to product.product_id
... (magazine-specific attributes)

web_site
========
web_site_id INT
name VARCHAR
product_id INT -> Foreign key to product.product_id
... (web-site specific attributes)

This way I do not need to make a huge table with a column for each attribute of different product types (most of which will then be NULL)

How do I SELECT a product by product.product_id and see all its attributes?
Do I have to make a query first to know what type of product I am dealing with and then, through some logic, make another query to JOIN the right tables? Or is there a way to join everything together? (if, when I retrieve the information about a product_id there are a lot of NULL, it would be fine at this point).

Thank you

Answer 1

You could do it all in one query, a few columns would stay empty:

SELECT
  t.product_type_name,
  t.product_type_id 
  p.product_id,
  p.product_name,
  p.[common attributes to all products...],
  m.*,
  w.*
FROM
  product p
  INNER JOIN product_type t ON t.product_type_id = p.product_type_id
  LEFT  JOIN magazine     m ON m.product_id      = p.product_id
  LEFT  JOIN web_site     w ON w.product_id      = p.product_id
WHERE
  p.product_id = ?

Use product_type_id in your app to determine which columns of the result set are interesting to you in any particular case.

As far as performance goes, this should run pretty quickly (foreign keys, indexes); and it produces a consistent result set for any product type.

I would recommend against using .* and for explicitly listing every column name, this is more portable, more maintainable and less error-prone.