EXPLAIN EXTENDED

A friend of mine who owns a stock photo website once told me he had designed an awesome photo auto-tagging algorithm which was 80% correct: just tag all photos cats and have four hits out of five.

But seriously, how do we calculate most used tags on photos?

For those not familiar with different tagging models, I highly recommend this old but very useful article: Tags: Database schemas.

Now, in this tutorial, we'll use the toxi approach to tagging. Later in the article I'll explain why we would choose that approach over the others.

Let us create a simple schema which would allow us to store, tag and sell photos:

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You're building a social network and want to store who is friends with whom. OK. Let's assume you want Facebook-style friends: by agreeing to be friends with someone you allow them to be friends with you too. This is called a symmetric relationship and if your social model sticks to that kind of relationship, your application should enforce its symmetry.

How shall we store friends in a database table? There are several options.

Initiator first

A table with one record per friendship and two fields for each of the friends. The person who requested the friendship is stored in the first field:

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