7. ENTITY RELATIONSHIP MODELLING
We characterize an attribute as being single-valued if there is only one value at a given time for the attribute.
Consider the Employee entity type for a typical business application where we need to include a gender attribute. Each employee is either male or female, and so there is only one value to store per employee. In this case, we have an attribute that is single-valued for each employee. Single-valued attributes are shown with a simple oval as in all diagrams up to this point. In all of our examples so far, we have assumed that each attribute was single-valued.
1) Consider a marriage entity type and attributes marriage date, marriage location, husband, wife. Each marriage will only have one value for each of these attributes. Illustrate the marriage entity and its single-valued attributes in an ERD.
2) A college or university will keep track of several addresses for a student, but each of these can be named differently: for example, consider that a student has a mailing address and a home address. Create an ERD for a student entity type with two composite attributes for student addresses where each comprises several single-valued attributes.