SQL field with multiple id's of other table

Question

Could someone give me an idea how to create this database structure. Here is an example:

Table \"countries\":
id, countryname
1, \"US\"
2, \"DE\"
3, \"FR\"
4         


        

Answer 1

You need an intersection table for that many-to-many relationship.

Table Country
CountryID, CountryName

Table CountryProduct
CountryID, ProductID

Table Product
ProductID, ProductName

You then Inner Join all 3 tables to get your list of Countries & Products.

Select * From Country 
Inner Join CountryProduct On Country.CountryID = CountryProduct.CountryID 
Inner Join Product On CountryProduct.ProductID = Product.ProductID

Answer 2

the best approach for relational databases is the following :

One table for coutries, let's say country_id, country_desc (country_id is primary)

one table for products, let's say product_id, product_desc and as many columns as you want (product_id is primary)

if you had only one country for sure, it'd be enough to have a foreign key pointing to country_id in each product row. Having a foreign key asserts that there is an actual country behing a country_id referring to country table.

In your case you have several countries for a product, so add a separate association table product_id, country_id

both keys primary and both foreign as well.

Answer 3

Without denormalizing, you'll need to add an extra table

Table Product countries
ProductID CountryID
1         1
1         2
1         4...

Answer 4

You could also make a third table countries_products with fields country_id and product_id.

Answer 5

What you're talking about is normalisation. You have a many-to-many structure, so you should create another table to link the two. You should never (ok, pretty much never) use delimited strings to store a list of values in a relational database.

Here's an example of the setup:

product_countries table

productid | countryid
----------+-----------
1         | 1
1         | 2
1         | 4
2         | 2
2         | 3
2         | 4

You can use a foreign key to each other table, then make them both into a composite primary key.

You can then get a list of supported products for a country ID like this:

SELECT * FROM products, product_countries
WHERE products.id = product_countries.productid
AND product_countries.countryid = $cid