Entity-Relationship Data Model N. Harika Lecturer(csc)
Transcript of Entity-Relationship Data Model N. Harika Lecturer(csc)
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Entity-Relationship Data Model
N. HarikaLecturer(csc)
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Presentation Overview• Short historical overview• Elements of E-R Model• Basic organization & relationships in E-R Model• Design principles
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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 35th most sited paper in computer science
• Chen has written papers interconnecting E-R model and linguistics
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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
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Elements of E-R ModelData represented graphically via entity-
relationship diagram which contains 3 main element types:Entity setsAttributesRelationships
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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
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E-R Diagram
• Represents E-R elements by nodes of specific shape to indicate kindo Entity sets are represented by rectangleso Attributes are shown as ovalso Relationships correspond to diamonds
• Simple example from the book:
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Simple illustration
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INSTANCES OF E-R DIAGRAM 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
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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
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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.
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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
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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
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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
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