IST223 Crib sheet

rectangles, and relationships are shown by lines between the rectangles. Attributes are generally listed within the rectangle. The many side of many relationships is represented by a crows footentity-relationship (E-R) modelA set of constructs and conventions used to create data models. The things in the users world are represented by entities, and the associations among those things are represented by relationships. The results are usually documented in an entity-relationship (E-R) diagramID-dependent entityan entity whose identifier includes the identifier of another entityidentifierwhich are attributes that name, or identify, entity instancesidentifying relationshipIn such relationships, the parent is always required, but the child (the ID-dependent entity) may or may not be required, depending on application requirements. Identifying relationships are shown with solid lines in E-R diagrams.is-aRelationships among supertype/subtype entitiesmandatoryat least one entity instance must participate in the relationshipmaximum cardinalityThe maximum cardinality is the maximum number of entity instances that can participate in a relationship instance.minimum cardinalityThe minimum cardinality is the minimum number of entity instances that must participate in a relationship instance.nonidentifying relationshiprelationship drawn with a dashed line (refer to Figure 5-7) is used between strong entities and is called a nonidentifying relationship because there are no ID-dependent entities in the relationship.null valueare a problem because they are ambiguous. They can mean that a value is inappropriate, unknown, or known, but not yet been entered into the databaseparentAn entity or row on the one side of a one-to-many relationshiprecursive relationshipoccurs when an entity type has a relationship to itself.relationship classAssociations among entity classesrelationship instanceassociations among entity instances.strong entityan entity that represents something that can exist