SQL field with multiple id's of other table - sql

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, "IT"
Now I have another table "products" and in there I would like to store all countries where this product is available:
Table "products":
id,productname,countries
1,"product1",(1,2,4) // available in countries US, DE, IT.
2,"product2",(2,3,4) // available in countries DE, FR, IT.
My question:
How do I design the table structure in "products" to be able to store multiple countries?
My best idea is to put a comma-separated string in there (i.e. "1,2,4"), then split that string to look up each entry. But I doubt that this the best way to do this?
EDIT: Thank you all for your help, amazing! It was difficult to choose the right answer,
I finally chose Gregs because he pointed me to a JOIN explanation and gave an example how to use it.

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

Without denormalizing, you'll need to add an extra table
Table Product countries
ProductID CountryID
1 1
1 2
1 4...

Answer 3

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

Answer 4

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

Answer 5

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.