Description of the collaborative filtering illustration

This figure shows SQL-based collaborative filtering. as described in text before the figure.

The figure itself shows circles representing four customers (C1, C2, C3, and C4) on the left, and five items (I1, I2, I3, I4, and I5) on the right. Arrows go from each customer to one or more items, each arrow indicating that the customer has given a rating to an item. For example, customer C1 has rated three items (I1, I2, and I4), whereas customer C3 has only rated one item (I3).