Entity sets and the relationship set borrower

Info icon This preview shows pages 12–15. Sign up to view the full content.

View Full Document Right Arrow Icon
entity sets and the relationship set borrower . borrower Figure: Symbol for derived attribute in E-R diagram. A A Figure: Symbol for multivalued attribute in E-R diagram.
Image of page 12

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
9 The association between entity sets is referred to as participation ; that is, the entity sets E 1 , E 2 , ..., E n participate in relationship set R . A relationship instance in an E-R schema represents an association between the named entities in the real-world enterprise that is being modeled. As an illustration, the individual customer entity Hayes, who has customer identifier 677-89-9011, and the loan entity L-15 participate in a relationship instance of borrower . This relationship instance represents that, in the real-world enterprise, the person called Hayes who holds customer-id 677-89-9011 has taken the loan that is numbered L-15. A relationship may also have attributes called descriptive attributes . For example, consider the entity sets customer and loan and the relationship set borrower . We could associate the attribute date-issued to that relationship to specify the date when the loan was issued: Most of the relationship sets in a database system are binary – that is, they involve two entity sets. Occasionally, however, relationship sets involve more than two entity sets. As an example, consider the entity sets employee , branch , and job . Examples of job entities could include manager, teller, auditor, and so on. Job entities may have the attributes title and level . The relationship set works-on among employee , branch , and job is an example of a ternary relationship. A ternary relationship among Jones, Perryridge, and manager indicates that Jones acts as a manager at the Perryridge branch. Jones could also act as auditor at the Downtown branch, which would be represented by another relationship. Yet another relationship could be between Smith, Downtown, and teller, indicating Smith acts as a teller at the Downtown branch. The number of entity sets that participate in a relationship set is called the degree of the relationship set. A binary relationship set is of degree 2; a ternary relationship set is of degree 3. Constraints An E-R enterprise schema may define certain constraints to which the contents of a database must conform. Different types of constraints can be depicted in E-R model: 1. Cardinality Constraints – one-to-one , one-to-many , many-to-one , many-to-many 2. Participation Constraints – partial , total 3. Key Constraints – superkey , candidate key , primary key employee employee-name employee-id job level title Figure: Ternary relationship. works-on branch branch-name branch-city branch-street customer customer-street customer-name customer-city customer-id loan amount loan-no Figure: Descriptive attribute date-issued . borrower date-issued
Image of page 13
10 Cardinality Constraints / Mapping Cardinalities / Cardinality Ratios Cardinality constraints express the number of entities to which another entity can be associated via a relationship set.
Image of page 14

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 15
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern