ERD For a full discussion and more examples please consult: http://www.library.cornell.edu/cts/elicensestudy/dlfdeliverables/fallforum2003/ERD_final.doc Also search the WWW for more resources on ERD
Entities • So far, no relationship between students and schools has been indicated.
Relationships now? Read the first relationship definition, “attends,” when tracing the relationship left to right or top to bottom. Read the second definition, “enrolls,” when tracing the relationship right to left or bottom to top
A little more descriptive…Optionality and Cardinality “Optionality” expresses whether the relationship is optional or mandatory. “Cardinality” expresses the maximum number of relationships. (first the optional aspect of the relationship, second the cardinal aspect)
What’s happening here… a supplier may provide many different products, and each type of product may be offered by many suppliers
Brain Teaser…? Is this possible?
Brain Teaser…? Recursive Relationship Is this possible? Yes… how? The diagram indicates that a person may be the father of zero or many persons (children), and that a person may have zero or one father. (Not every person’s father will be recorded in the system, so the relationship is modeled as optional).
How about this one…? Exclusive-Or Relationship Each convict is assigned to a prison, or to a parole officer, but not both