Link to multiple foreign rows from one row - mysql

Question

I really don't know how this is called so I couldn't find an answer.
For example I have the following tables:
Table products Table users
+----------+----------+----------+ +----------+----------+
| id | name | price | | username | products |
+----------+----------+----------+ +----------+----------+
| 1 | Bread | 1.5 | | James | 1, 2 |
+----------+----------+----------+ +----------+----------+
| 2 | Cookies | 2.0 | | David | 2, 3 |
+----------+----------+----------+ +----------+----------+
| 3 | Milk | 1.2 | | Paul | 3, 1 |
+----------+----------+----------+ +----------+----------+
products.id is the foreign key for users.products.
I would like to get the name and price of all David's products using a MySQL query. This should return the following:
+----------+----------+
| name | price |
+----------+----------+
| Cookies | 2.0 |
+----------+----------+
| Milk | 1.2 |
+----------+----------+
The JOIN function would be the best to use I guess, but David can have just 1 or 100 products. Creating 100 columns to fit all products doesn't sound very efficient.
How can I realize this?
Currently I'm just fetching everything and filter it using PHP, this is not a problem at the moment but as the tables will grow I guess this will be very inefficient.

Answer 1

This could be solved by changing your data model.
users
+----------+----------+
| id | username |
+----------+----------+
| 1 | Fred |
+----------+----------+
| 2 | John |
+----------+----------+
products
+----------+----------+----------+
| id | name | price |
+----------+----------+----------+
| 1 | Bread | 1.5 |
+----------+----------+----------+
| 2 | Cookies | 2.0 |
+----------+----------+----------+
| 3 | Milk | 1.2 |
+----------+----------+----------+
And here comes the magic: You could connect the two tables using a third table:
user_procuct_connections
+----------+----------+------------+
| id | user_id | product_id |
+----------+----------+------------+
| 1 | 1 | 2 | -> Fred has Cookies
+----------+----------+------------+
| 2 | 1 | 3 | -> Fred also has Milk
+----------+----------+------------+
| 3 | 2 | 1 | -> John has Bread
+----------+----------+------------+
If you want a user to be able to own a single product only, then you can remove the id column, an make the user_id and product_id primary key together.
Then when you want to get for example all of Freds products then just
SELECT
*
FROM
products
WHERE
id IN (
SELECT
product_id
FROM
user_procuct_connections
WHERE
user_id = 1
)

Answer 2

You could try this:
SELECT * FROM products pt
where FIND_IN_SET(pt.id,(select us.prices from users us
WHERE us.username = "David"));
Working fiddle: http://sqlfiddle.com/#!2/4f78d/2

Answer 3

The design for the 'Users' table is wrong. Instead of working around this bad design, please change the design.
So effectively, you design could be as such:
Table Products :
ID Name Price
1 Bread 1.5
2 Cookies 2.0
3 Milk 1.2
Table users :
Username Products
James 1
James 2
David 2
David 3
Paul 3
Paul 1
You see, you can have multiple rows for each user i.e.one row for each product in the Users table. You can maintain another Boolean field to identify which ones are currently 'active' or 'applicable'. Joins' would be much easier in that case. Also, updating records would also be easier. In case, in future you'd want to retrieve & analyse data from historic records, that would be possible too!!!
Another thing with the current design is that times and again you'd have to work around the infamous "comma-seperated values". Let's say you have a record as such:
Username Products
James 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
If you need to sort this data into rows, you'd have to work-around the 'Comma-seperated' values so many times. Imagine doing that for 'n' number of records!!
Hope this helps!!!

Answer 4

If you join the tables like:
SELECT name, price FROM products AS p INNER JOIN user_products AS up ON up.id = p.product_id AND up.user_id = <davids id>
You will get an array where first column will be the name and the second the price. You can then do what you want with it in PHP and change the array. You don't have to filter anymore.