24_EntityRelationship3.pdf - CITS1402 Relational Database...

This preview shows page 1 - 8 out of 19 pages.

CITS1402 Relational Database Management Systems Video 24 — Entity Relationship Diagrams III Gordon Royle Department of Mathematics & Statistics
ERD We’ll finish the basic features entity-relationship diagrams, and look briefly at ERDPlus.com .
Classic Models The relationship is that “customers place orders”.
Relationships How relationships are implemented in the database depends on how the business logic constrains the relationship. These constraints are usually called cardinality constraints of the relationship, and are included in an ERD through specific symbols.
An example In ClassicModels, the table orderdetails records connections between products and orders . 10100 10101 10102 10103 10104 products . . . S12_3990 S18_2248 S50_1341 S50_4713 S700_1691 orders . . .
The relationship 10100 10101 10102 10103 10104 products . . . S12_3990 S18_2248 S50_1341 S50_4713 S700_1691 orders . . .
Cardinality Cardinality is a fancy word for the “size” or “number” of something. Restrictions on how many times an element (of either entity set) can occur in a relation are called cardinality constraints .

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture