Unique identifier across multiple tables? - mysql

Question

Unique identifier across multiple tables?
Using MySQL, I'm trying to associate products with a purchase order.
Those products can become from different tables probably with different schema's.
I cant associate because of the identity of each product by schema, not by all kinds of products.
Cellphone
+-----+--------+---------+-------+----------+-------+
| id | brand | version | color | capacity | price |
+-----+--------+---------+-------+----------+-------+
| 1 | iphone | 5s | black | 16 | 9500 |
| 2 | iphone | 5s | white | 32 | 10000 |
| 3 | iphone | 5s | blue | 32 | 10000 |
+-----+--------+---------+-------+----------+-------+
Course
+-----+-----------+------------+-------+
| id | topic | idschedule | price |
+-----+-----------+------------+-------+
| 1 | Photoshop | 1 | 9500 |
| 2 | HTML5 | 2 | 10000 |
| 3 | CSS3 | 3 | 10000 |
+-----+-----------+------------+-------+
I've made a table Product, that together with a product identity, produce an unique identifier.
Product
+-----+-----------+
| id | Schema |
+-----+-----------+
| 1 | Cellphone |
| 2 | Course |
+-----+-----------+
ProductEspecification
+-----+-----------+------------------+----------+------------+
| id | idproduct | idespecification | quantity | idpurchase |
+-----+-----------+------------------+----------+------------+
| 1 | 1 | 1 | 10 | 1 |=> 10 iphones
| 2 | 2 | 3 | 1 | 1 |=> 1 CSS3 course
+-----+-----------+------------------+----------+------------+
Is there a better approach?

Answer 1

You need to use abstraction and Table Inheritance.
An abstract thing you sell (good, service, etc) is called a Product.
An abstract physical good you sell is called a Good. Good inherits Product.
An abstract service you offer is called a Service. Service inherits Product.
Cellphone inherits Good.
Course inherits Service.