How to use foreign keys with PHP

Question

So I understand how to create foreign keys and I know what is the purpose of the FK. But I have a problem in understanding How to use them. I asked a question regarding Fore

Answer 1

Instead of reading so many links, just try to implement this in any simple project. I'm just explaining how we gonna use the above tables.

Suppose you 3 users in user table and 5 items in items table.

user table

id  |  username |  password 
 1      abc         123
 2      def         456
 3      qwe         987 

items table

i_id  |  name   |  price 
 1      item 1    6
 2      item 2    8
 3      item 3    11 
 4      item 4    3
 5      item 5    14  

your user_purchase table look like this

CREATE TABLE user_purchase( i_id INT(11) NOT NULL, id INT(11) NOT NULL, FOREIGN KEY (i_id) REFERENCES items(i_id), FOREIGN KEY (id) REFERENCES user(id) );

There is no need of item name again in this table. So I have removed.

i_id |  id  
 1      1      
 1      2
 2      2      
 3      3      

In the above table we will get, user 1 has purchased item 1, user 2 has purchased item 1,item 2 and user 3 has purchased item 3.

This is how normalization works. You can use MySQL JOIN for getting user name and item details

SELECT B.user_name,C.name AS item_name,C.price
FROM user_purchase A 
JOIN user B ON A.id = B.id
JOIN items C ON A.i_id = C.i_id

Here is foreign key use to join

A.id = B.id
A.i_id = C.i_id