Dbms chapter iv
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Transcript of Dbms chapter iv
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Subject Name Code Credit HoursDatabase System COMP 219 3
Chapter IV
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Subject Name Code Credit HoursDatabase System COMP 219 3
Data Models• A collection of tools for describing
– Data – Data relationships– Data semantics– Data constraints
• Relational model• Entity-Relationship data model (mainly for database design) • Object-based data models (Object-oriented and Object-relational)• Semistructured data model (XML)• Other older models:
– Network model – Hierarchical model
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Subject Name Code Credit HoursDatabase System COMP 219 3
Data Models
• A collection of Conceptual tools for describing – Data – Data relationships– Data semantics– Data constraints
• Different data Models• - Relational model - Entity-Relationship data model (Conceptual Model-mainly for database design)
- Object-based data models (Object-oriented and Object-relational) - Semi structured data model (XML)• Other older models:
– Network model – Hierarchical model
A Data model is a set of concepts that can be used to describe the structure of the db.
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Subject Name Code Credit HoursDatabase System COMP 219 3
Entity-Relationship data model
• It is a high level conceptual data model that describes the structure of db in terms of entities, relationship among entities & constraints on them..
• Basic Concepts of E-R Model: - Entity- Entity Set- Attributes- Relationship- Relationship set- Identifying Relationship
I
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Subject Name Code Credit HoursDatabase System COMP 219 3
Entity-Relationship data model
• Entity:-It is a an object that exists in the real world.
• Example: - Person, Employee, Car, Home etc..
Object with conceptual Existence - Account, loan, job etc…
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Subject Name Code Credit HoursDatabase System COMP 219 3
Entity-Relationship data model
• Entity Set: - A set of entities of the same type.• Attributes:
- A set of properties that describe an entity.
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Subject Name Code Credit HoursDatabase System COMP 219 3
• Types of Attributes:• Simple (or) atomic vs. Composite:• - An attribute which cant be sub divided. (Eg.Age)• - An attribute which can be divided into sub parts is called • as composite attribute.
e.g.. Address- Apartment no. - Street
- Place - City
- DistrictSingle Valued vs. Multivalued:• -An attribute having only one value (e.g.. Age,eid,sno) • - An attribute having multiple values (e.g.. Deptlocat- A dept can be located in
several places)
Entity-Relationship data model
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Subject Name Code Credit HoursDatabase System COMP 219 3
Entity-Relationship data model
• Stored Vs Derived• - Stored attribute is one that has some value where as
derived attribute is a one where its value is derived from sa.• -E.g.. SA-DOB• DA- Age derived from DOB.• Key Attribute:• - An attribute which is used to uniquely identify records.• E.g.. eid, sno, dno
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Subject Name Code Credit HoursDatabase System COMP 219 3
Entity-Relationship data model
• Relationship:• - It is an association among several
entities. It specifies what type of relationship exists between entities.
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Subject Name Code Credit HoursDatabase System COMP 219 3
• Relationship set:• - It is a set of relationships of the same type.
Entity-Relationship data model
1 AA 1000
2 BB 2000
100 FFF 10000
1 AA AC
2 BB AD
100 FFF SD
EntityENTITY SET
DEPT. ENTITY TYPERelationship SetEmp.. ENTITY TYPE
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Subject Name Code Credit HoursDatabase System COMP 219 3
• Weak Entity Set:• - No key attributes.• Identifying Relationship:• - The relationship associated with the weak
entity type
Entity-Relationship data model
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Subject Name Code Credit HoursDatabase System COMP 219 3
Constraints
• Two of the most important constraints are
• a. Mapping Constraints• b. Participation constraints
»Participation constraints
Total Participation Partial Participation
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Subject Name Code Credit HoursDatabase System COMP 219 3
a. Mapping Cardinalities:Mapping Cardinalities OR CARDINALITY RATIOS, EXPRESSS THE NUMBER OF
ENTITIES TO WHICH ANOTHER ENTITY CAN BE ASSOCIATED VIA A RELATIONSHIPSET.
• Several types of Mapping Cardinalities. They are,• a.i. One-to-One• An entity in set A is associated with at most one entity in set B and
vice versa.
e1
e2
e3
d1
d2
d3
Employee Dept.Works for
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Subject Name Code Credit HoursDatabase System COMP 219 3
• a.i. One-to-many• An entity in set A is associated with zero or more
no. of entities in set B and an entity in B is associated with at most one entity in A.
a. Mapping Cardinalities:
e1
e2
e3
d1
d2
d3
Employee Dept.Works for
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Subject Name Code Credit HoursDatabase System COMP 219 3
a. Mapping Cardinalities: a.i. Many-to-One One or more no. of entities in set A is associated with at
most one entity in B. An entity in B can be associated with any no. of entities in A.
e1
e2
e3
e4
d1
d2
d3
Employee Dept.Works for
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Subject Name Code Credit HoursDatabase System COMP 219 3
• a.i. Many-to-Many• One or more no. of entities in set A
is associated with one or more no. of entities in set B.
a. Mapping Cardinalities:
e1
e2
e3
e4
d1
d2
d3
Employee Dept.Works for
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Subject Name Code Credit HoursDatabase System COMP 219 3
• The participation of an entity set E in a relationship set R is said to be total if every entity in E participates in atleast one relationship in R.
b. Participation Constraints:Total Participation
Partial Participation:The participation of an entity set E in a relationship set R is said to be partial if only a few of the entities in E participated in relationship in R.
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Subject Name Code Credit HoursDatabase System COMP 219 3
b. Participation Constraints:
• E.G..
Partial participation Total participation
EmployeeDept.
Manages