By Rosemary Rock-Evans
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On the Data Model, extra properties of the Entitles and Relationships are shown using special symbols. These are: • • • • • Exclusivity Optionallty of a Relationship Degree (or cardinality) of a Relationship Entity sub-types Identified by How to do Data Modelling 3·1 INTRODUCTION This chapter shows you how to get the Data Modelling deliverables - the Data Model and all the related Information from a number of well defined Inputs'. These Inputs are those you would expect to get from the collection task (Interviews, observation, meetings, study of existing documentation).
4 Example 1 - Step 1 Course No. Course Name No. 5 Example 1 - Step 2 COURSE 'EVENT OR COURSE •GIVING' COURSE Course No. Course Name No. 6 Example 1 - Step 3 The data Items are split, and one-to-many Relationships are created. 5. Course is an obvious choice for the first group of items, but the second group poses a problem. What is being described? The start and end date give the clue. g. Basic Systems, so the group of four data items describe an actual giving of the course. 6. How to do Data Modelling 59 4.
It is important that we create an Attribute to enable our sub-types to be identified. 21 shows the Attributes of the Entity 'Member' and some occurrences. An Attribute Type of Member has been created to describe the sub-types as a whole and a code created for each sub-type - f = full; d = day; s = student. The table also shows another important property of Entity sub-types - some Attributes may be applicable to only one sub-type. In the example, Hours of Play is applicable only to Day Members. This is important in design as this knowledge will help u s to decide, depending on the power of our DBMS, whether we will need variable length records or compression, or have to split records to save wasted space.