i)Entities and Attributes –
Entity - An entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity.
Entity - An entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity.
ii) an attribute is a characteristic. In a database management system (DBMS), an attribute refers to a database component, such as a table. It also may refer to a database field. Attributes describe the instances in the row of a database.
ii)Composite and Atomic attributes –
Composite attribute −
Composite attributes are made of more than one simple attribute. For example, a student's complete name may have first_name and last_name.
Simple attribute (Atomic) −
Simple attributes are atomic values, which cannot be divided further. For example, a student's phone number is an atomic value of 10 digits.
iii)Single valued and Multivalued attributes –
• Single-value attribute −
Single-value attributes contain single value. For example − Social_Security_Number.
•Multi-value attribute −
Multi-value attributes may contain more than one values. For example, a person can have more than one phone number, email_address, etc.
iv)Attribute Defined Specialization , Predicate Defined Specialization and User Defined –
Attribute defined specialization -If all the subclasses in a specialization have their membership condition on the same attribute of the super class, the specialization itself is called an attribute defined specialization, and the attribute is called defining attribute of the specialization. We display an attribute defined specialization by placing the defining attribute name next to the arc from the circle of the super class as shown in the figure.
User defined specialization -
When we do not have a condition for determining membership in a subclass, the subclass is called user defined specialization. Membership in such a subclass is determined by the database user when they apply the operation to add an entity to the subclass. Membership is specified individual for each entity by the user, not by the condition that may be evaluated automatically.
Predicate defined specialization -
In some specialization we can determine exactly the entities that will become member of each subclass by placing a condition on the value of some attribute of the super class. Such subclasses are called predicate defined (or condition defined) subclasses.
v)Total and Partial specialization –
Partial specialization -
If an EMPLOYEE entity does not belong to any of subclasses, the specialization is called partial specialization which is shown by single line. For example some EMPLOYEE entities do not belong to any of the subclasses {SECRETARY, ENGINEER, TECHNICIAN} then that specialization is partial.
Total specialization -
A total specialization constraint specifies that every entity in the super class must be the member of at least one subclass in the specialization. For example if every employee must be either an HOURLY_EMPLOYEE or SALARIED_EMPLOYEE, then the specialization {HOURLY_EMPLOYEE, SALARIED_EMPLOYEE} is a total specialization of EMPLOYEE.
vi) Disjoint and Overlapping Constraints –
Disjoint constraint -
This constraint specifies that the subclasses of the specialization must be disjoint. This means that an entity can be a member of at most one of the subclasses of the specialization. A specialization that is attribute-defined implies the disjoint ness constraint if the attribute used to define the membership predicate is single valued. It is displayed by placing d in the circle
Composite attributes are made of more than one simple attribute. For example, a student's complete name may have first_name and last_name.
Simple attribute (Atomic) −
Simple attributes are atomic values, which cannot be divided further. For example, a student's phone number is an atomic value of 10 digits.
iii)Single valued and Multivalued attributes –
• Single-value attribute −
Single-value attributes contain single value. For example − Social_Security_Number.
•Multi-value attribute −
Multi-value attributes may contain more than one values. For example, a person can have more than one phone number, email_address, etc.
iv)Attribute Defined Specialization , Predicate Defined Specialization and User Defined –
Attribute defined specialization -If all the subclasses in a specialization have their membership condition on the same attribute of the super class, the specialization itself is called an attribute defined specialization, and the attribute is called defining attribute of the specialization. We display an attribute defined specialization by placing the defining attribute name next to the arc from the circle of the super class as shown in the figure.
User defined specialization -
When we do not have a condition for determining membership in a subclass, the subclass is called user defined specialization. Membership in such a subclass is determined by the database user when they apply the operation to add an entity to the subclass. Membership is specified individual for each entity by the user, not by the condition that may be evaluated automatically.
Predicate defined specialization -
In some specialization we can determine exactly the entities that will become member of each subclass by placing a condition on the value of some attribute of the super class. Such subclasses are called predicate defined (or condition defined) subclasses.
v)Total and Partial specialization –
Partial specialization -
If an EMPLOYEE entity does not belong to any of subclasses, the specialization is called partial specialization which is shown by single line. For example some EMPLOYEE entities do not belong to any of the subclasses {SECRETARY, ENGINEER, TECHNICIAN} then that specialization is partial.
Total specialization -
A total specialization constraint specifies that every entity in the super class must be the member of at least one subclass in the specialization. For example if every employee must be either an HOURLY_EMPLOYEE or SALARIED_EMPLOYEE, then the specialization {HOURLY_EMPLOYEE, SALARIED_EMPLOYEE} is a total specialization of EMPLOYEE.
vi) Disjoint and Overlapping Constraints –
Disjoint constraint -
This constraint specifies that the subclasses of the specialization must be disjoint. This means that an entity can be a member of at most one of the subclasses of the specialization. A specialization that is attribute-defined implies the disjoint ness constraint if the attribute used to define the membership predicate is single valued. It is displayed by placing d in the circle
Overlap constraint -
This constraint specifies that
the subclasses of the specialization are not constrained to be disjoint. This
means that an entity can be a member of more then one subclasses of the
specialization. This case, which is the default, is displayed by placing O in
the circle.
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