Entities, Attributes and Relationships

Designing before building

The entity-relationship (ER) model, introduced by Peter Chen in 1976, describes data at the conceptual level. An entity is a real-world object (a student, a course); an entity set is a collection of similar entities, drawn as a rectangle. Attributes describe entities, drawn as ovals: simple (roll number) or composite (name made of first and last), single-valued or multi-valued (phone numbers, double oval), stored or derived (age derived from date of birth, dashed oval), and the key attribute (underlined). A relationship connects entities (a student enrols in a course), drawn as a diamond, and may have its own attributes (the grade belongs to the enrolment, not to the student or the course). Cardinality constrains how many entities participate: one-to-one, one-to-many or many-to-many. Participation is total (every entity must take part, double line: every course must belong to a department) or partial (single line: not every student must hold a scholarship).

A small ER diagram

Entity sets, attributes and a many-to-many relationship with its own attribute.

Describing a university in ER terms

Textual notation for the diagram you would draw.

entity STUDENT   (roll_no [key], name (first, last) [composite], phones [multi-valued],
                  dob, age [derived])
entity COURSE    (course_id [key], title, credits)
entity DEPARTMENT(dept_id [key], name)

relationship ENROLS     STUDENT M --- N COURSE      attributes: semester, grade
relationship OFFERS     DEPARTMENT 1 --- N COURSE   COURSE participation: total
relationship HEADS      PROFESSOR 1 --- 1 DEPARTMENT

reading: a student enrols in many courses; a course has many students;
         every course is offered by exactly one department

Put attributes where they belong

A grade is not a property of a student (they have many grades) or of a course; it describes one student's enrolment in one course. Attributes of a relationship are a frequent exam and design question.

Quick check: In an ER diagram, how is a multi-valued attribute such as phone numbers drawn?

• A dashed oval
• A diamond
• A double oval
• An underlined rectangle