2 Developing an E-R Diagram The process of database design is an iterative rather than a linear or sequential process. It usually begins with a general narrative of the organization’s operations and procedures. The basic E-R model is graphically depicted and presented for review. The process is repeated until the end users and designers agree that the E-R diagram is a fair representation of the organization’s activities and functions.
3 employee department project dependent works for manages works on dependents of controls supervision 1 N N 1 NM 1 N 1 1 N 1 supervisor supervisee partial constraint total constraint
4 ER Modeling Entity Relationship Modeling is a process used to help us understand and document the informational requirements of a system. - a system at your university, college, or school needs to keep information about students including their names, addresses, birth dates, courses, enrollments, grades, etc. - these things are documented in the model - we create a drawing called an Entity Relationship Diagram, or ERD
5 Eventually: We create a physical model in some (relational) database management system (DBMS) We translate the ER Model into a relational database Suppose we know of four student entities and two courses entities. For example, four students named John, Amelia, Lee, and April, and two courses “Intro to Art” and “Intro to History”. Information about these is listed below.
6 Students NameId# Phone John184283-4984 Amelia337838-3737 Lee876933-2211 April901644-3838 Course NameCourse#Dept Intro to Art661Art Intro to History765History
7 set of students We can represent these two sets of entities using set diagrams. Intro to Art Intro to History John Amelia Lee April Relationships describe how entities relate to one another set of courses
8 Suppose we have the two courses and four students listed previously. Suppose also that - John and Amelia are enrolled in Intro to Art - John and Lee are enrolled in Intro to History - April is not enrolled in any course. Below, we depict a relationship set and show the four instances of an enroll in relationship.
9 John Amelia Lee April Intro to Art Intro to History
10 Note there are four instances of enroll in. Each Instance involves exactly two entities: a student entity and a course entity. Because each instance of enroll in involves two entities, it is referred to as a binary relationship. Binary relationships will be the focus of most of our discussions.
11 An example of an Entity Relationship Model: Entities are indicated using rectangular shapes, and relationships between entities are shown with a diamond shape on a line connecting the entities. StudentsCourse Enroll-in m n Entity sets are shown with a rectangular shape. Relationship sets are shown with a diamond shape. The "m" indicates that one course may have many students enrolled in it. The "n" indicates that one student may have enrolled in many courses.
12 The ER model was introduced by Peter Chen in his paper The Entity-Relationship Model--Toward a Unified View of Data (ACM Transactions on Database Systems, Vol. 1, No. 1, 1976). This paper is one of the most cited papers in the computer field, and has been considered one of the most influential papers in computer science.
14 name Derived attribute Mutlti-valued attribute Partial participation name Total participation 1 n N Cardinality name Composite attribute ERD symbols
15 1 N 1 1 1 N 1 M supervisee N employee department project dependent With attributes, etc: works for manages works on dependents of controls supervision bdate ssn name lname minit fname sex address salary birthdatenamesex relationship name numberlocation name numberlocation number of employees startdate hours supervisor degree N