Tell me about foreign keys.

[Greyfish]

Hey I was recently advised to use foreign keys in my MySQL database and I'm not very familiar with them. Can someone explain to me exactly what they do and how I query the database using them?

[shk]

A Foreign Key is a referential constraint between two tables. For instance:

Table 1: Article
article_id
article_title
article_body
(....)

Table2: Category
category_id
category_name
(...)

Primary Keys (PK) are to identify each entry (article_id, category_id). In order to know which article belongs to which category you need to link both tables.. .the way to do it is using foreign keys (FK) So it will look like these:

Table1: Article
article_id
article_title
article_body
article_category_id <-- (Foreign Key)
(....)

Table2: Category
category_id
category_name
(...)

So whenever you create an article you will need to store in Article table which category_id it belongs to as well.

So when you do the query

$sql="SELECT * FROM article WHERE article_category_id="1" ";

You will be able to show all the articles that belong only to category_id= 1

Hope it helped a little.

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