Entity-Relationship Data Model N. Harika Lecturer(csc)

Presentation Overview Short historical overview Elements of E-R Model Basic organization & relationships in E-R Model Design principles

HISTORY OF E-R MODEL E-R Model was proposed by Dr. Peter Chen (currently professor at Louisiana State University) Chen’s original paper on E-R Model is the 35 th most sited paper in computer science Chen has written papers interconnecting E-R model and linguistics

Introduction  Database Structure is often called Database Schema  E-R model is graphical in nature, thus making it easy to analyze and observe relationship between data elements  Most DBMS are based upon E-R model  E-R model is not a good match for the sophisticated data structures required in today’s DBMS

Elements of E-R Model Data represented graphically via entity-relationship diagram which contains 3 main element types: Entity sets Attributes Relationships

 Entity set  Is an abstract object, collection of such objects forms an entity set.  Similar notion as in OO design  Attribute:  Some concrete data (or object type) by which entity set is defined  Relationship  Specific connection between 2 or more entity sets Entity sets, Attributes, Relationships

E-R Diagram Represents E-R elements by nodes of specific shape to indicate kind o Entity sets are represented by rectangles o Attributes are shown as ovals o Relationships correspond to diamonds Simple example from the book:

Simple illustration

 DB described by E-R will contain specific data (i.e. database instance)  Each entity set will contain a particular finite set of entities  Each entity contains a particular value for each attribute  E-R data is not stored directly in DB

E-R Relationships  Suppose R is a relationship connecting entity sets E and F. Then:  If each member of E can be connected by R to at most one member of F, then we say that R is many-one from E to F. Note that in a many-one relationship from E to F, each entity in F can be connected to many members of E.  If R is both many-one from E to F and many-one from F to E, then we say that R is one-one. In a one-one relationship an entity of either entity set can be connected to at most one entity of the other set.  If R is neither many-one from E to F or from F to E, then we say R is many-many

MULTI-WAY RELATIONSHIPS  There is a relationship Sequel-of between the entity set Movies and itself.  To differentiate the two movies in a relationship, one line is labeled by the role Original and one by the role Sequel, indicating the original movie and its sequel, respectively.

RELATIONSHIPS CONTINUED  Some data models limit relationships to be binary  It is possible to convert multi-way relationship into a collection of binary many- one relationships  Need to introduce a connecting entity set, which will act as a bridge between smaller sets which come from splitting a larger multi-way relationship set.  Connecting entity set might have its own attributes

DESIGN PRINCIPLES Faithfulness: Design has to comply strictly with specifications Logical attributes and relationships Avoid redundancy "Everything should be made as simple as possible, but not simpler.” Albert Einstein Choose right relationships Select right elements Many choices exist between using attributes and using entity set/relationship combinations An attribute is simpler to implement than either entity set or a relationship

Design Principals Cont. To replace an entity set by an attribute or attributes of several entity sets 3 conditions must be enforced: o All relationships in which entity set is involved must have arrows entering it. That is, it must be the “one” in many-one relationships, or its generalization for the case of multi-way relationships. o The attributes for E must collectively identify an entity. if there are several attributes, then no attribute must depend on the other attributes o No relationship involves E more than once