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The ER model defines the conceptual view of a database. It works around real-world entities and the associations among them. At view level, the ER model is considered. 20 Chapter 6 Database Design and the E-R Model program course offerings dept title course courseno title credits syllabus prerequisite maincourse requires.

The ER model defines the conceptual view of a database. It works around real-world entities and the associations among them. Bijoux Terner Sport Wr30m Manual. At view level, the ER model is considered a good option for designing databases.

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. An entity set is a collection of similar types of entities. An entity set may contain entities with attribute sharing similar values. For example, a Students set may contain all the students of a school; likewise a Teachers set may contain all the teachers of a school from all faculties. Entity sets need not be disjoint.

Attributes Entities are represented by means of their properties, called attributes. Bitcomet Vs Utorrent Speed. All attributes have values. For example, a student entity may have name, class, and age as attributes.

There exists a domain or range of values that can be assigned to attributes. For example, a student's name cannot be a numeric value. It has to be alphabetic. A student's age cannot be negative, etc.

Types of Attributes • Simple attribute − Simple attributes are atomic values, which cannot be divided further. For example, a student's phone number is an atomic value of 10 digits. • 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.

• Derived attribute − Derived attributes are the attributes that do not exist in the physical database, but their values are derived from other attributes present in the database. For example, average_salary in a department should not be saved directly in the database, instead it can be derived.

For another example, age can be derived from data_of_birth. • 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. These attribute types can come together in a way like − • simple single-valued attributes • simple multi-valued attributes • composite single-valued attributes • composite multi-valued attributes Entity-Set and Keys Key is an attribute or collection of attributes that uniquely identifies an entity among entity set. For example, the roll_number of a student makes him/her identifiable among students. • Super Key − A set of attributes (one or more) that collectively identifies an entity in an entity set.

• Candidate Key − A minimal super key is called a candidate key. An entity set may have more than one candidate key. Thinkvantage Ultranav Driver - The Best Software For Your.