IST223 Crib sheet

Topics: Entity-relationship model, Entity, Relationship Pages: 5 (3425 words) Published: November 1, 2014
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 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 on its ownsubtypeIn generalization hierarchies, an entity or object that is a subspecies or subcategory of a higher-level type, called a supertype. For example, ENGINEER is a subtype of EMPLOYEE.supertypeIn generalization hierarchies, an entity or object that logically contains subtypes. For example, EMPLOYEE is a supertype of ENGINEER, ACCOUNTANT, and MANAGER.ternary relationshipA relationship between three entitiesunnormalizedweak entityIn an entity-relationship model, an entity whose logical existence in the database depends on the existence of another entity. All ID-dependent entities are weak, but not all weak entities are ID-dependent.1.1. _23__ composite identifier 1.2. _1821__ denormalize 1.3. _28__ discriminator 1.4. _11__ ID-dependent entity 1.5. _15__ is-a 1.6. __9_ minimum cardinalities 1.7. _16__ null value 1.8. _27__ recursive relationship 1.9. __32_ supertype 1.10. _18__ unnormalized 1.1. _7__ cardinality 1.2. _34__ crows foot 1.8. _8__ strong entity 1.9. __14_ ternary relationship 1.10. _30__ weak entity 1.5. _26__ mandatory 1.7. __4__ Sub-type 1 - all the entities in the entity set EMPLOYEE, ENGINEER, ACCOUNTANT 2 - the mapping from one address to another 3 - EMPLOYEE in the entity set EMPLOYEE, ENGINEER, ACCOUNTANT 4 - ACCOUNTANT in the entity set EMPLOYEE, ENGINEER, ACCOUNTANT 5 - an attribute that is a key of one or more relations other than the one in which it appears 6 - a domain definition that stands alone 7 - the maximum or minimum number of elements allowed on each side of a relationship 8 - an entity whose existence does not depend on the existence of any other entity 9 - M-O, O-M, M-M, O-O 10 - relationships that involve boxcar entities 11 - an entity whose identifier contains the identifier of a second entity 12 - the number of entities participating 13 - an identifier that combines a primary key and a foreign key 14 - three entities in a relationship 15 - relation between supertype and subtype entities 16 - an attribute value that...
Continue Reading

Please join StudyMode to read the full document

You May Also Find These Documents Helpful

  • Essay about cheat sheet
  • Essay on Finance-Crib Sheet
  • Globalisation Crib Sheet Essay
  • training sheet Essay
  • balance sheet Essay
  • formula sheet Essay
  • Essay on formula sheet
  • Sql-Cheat-Sheet Research Paper

Become a StudyMode Member

Sign Up - It's Free