Studierende stehen vor dem LC und blicken lächelnd einer Kollegin mit einer Mappe in der Hand nach.

Exercise No. 24: Paperboy (dfm)

In this simplified E/R schema a customer gets a newspaper by a paperboy. The customer lives in a certain district which belongs to a certain city. The newspaper is published by a certain publishing house which can belong to a media group. The time dimension consists of day, week and month.

Figure D.E.50.1 - paperboy

Please identify the fact of interest and build the attribute tree. Some attributes are not of interest for this data warehouse: We are not interested in the date of birth of a paperboy. Other attributes cannot be used for aggregation in a meaningful way. Those should be tagged as non-dimension attributes in the fact schema.

Then derive the fact schema from the tree by defining dimensions, fact attributes and hierarchies and build a glossary for the fact attributes if there are any.

Solution

Entity GETS is chosen as the only fact. Its identifier becomes the root of the attribute tree. All other attributes or identifiers become nodes.

Figure D.E.50.2 - The attribute tree

Customer, newspaper and paperboy are defined as dimensions, dimension day is introduced as a range of the date attribute. Time of delivery becomes the only fact attribute; remember number of customers is non-additive, using the "average" operator. The media group is tagged as optional.

The address of a customer cannot be used for aggregation and is therefore tagged as a non-dimension attribute.

Figure D.E.50.3 - The fact schema

time of delivery = AVG(GETS.time of delivery)

This exercise is part of a case study: dfm - apa - log

Exercises List