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E.R Digram tutorial

E.R Digram tutorial - Notes of ENTITY RELATIONSHIP...

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Notes of ENTITY RELATIONSHIP Modelling 1 Mrs Mousmi Ajay Chaurasia,Lect. Deptt. Of Information Technology, BIT DURG Entity – Relationship(ER) Modelling ER Model Concepts --Entities and Attributes -- Entity Types, Value Sets, and Key Attributes --Relationships and Relationship Types -- Weak Entity Types 1 --Roles and Attributes in Relationship Types A database can be modeled as: a collection of entities, relationships among entities. Entity/Relationship approach - one of the most well known modeling methods Developed by P.Chen in 1976 - many variations since then Entity : A real-world object that can be distinctly identified may represent some real physical object. E.g., Satish is a government employee; Kareena Kapoor is an actress; my car is a Honda City May represent some conceptual idea E.g., SC304 is a course; Semester 1 2001/2002 is a semester An entity is an object that exists and is distinguishable from other objects. Example: specific person, company, event, plant An entity set is a set of entities of the same type that share the same properties. Example: set of all persons, companies, trees, holidays. An Entity should be: An Object that will have many instances in the DB An Object that will have multiple attributes An Object that v r trying to model An Entity should not be: User of the DB An O/P of the DB(eg. Report) Attributes: An entity is represented by a set of attributes, that is,descriptive properties possessed by all members of an entity set.(value from corresponding entity) Example: customer = (customer-name, social-security, customer-street, customer-city) account = (account-number, balance) Domain – the set of permitted values for each attribute Attribute types: Simple and composite attributes. Single-valued and multi-valued attributes Null attributes Derived attributes Identifiers(Key) attributes
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Notes of ENTITY RELATIONSHIP Modelling 2 Mrs Mousmi Ajay Chaurasia,Lect. Deptt. Of Information Technology, BIT DURG Simple versus composite Simple attributes are atomic.E.g. tel_ph; house color; basic-salary; Composite attributes made up of simple attributes. E.g., address = (St_add,city,state, postal code) Composite Simple Key is underlined Single valued versus multivalued Single valued: single value associated with an attribute Multivalued: may have more than one values. E.g., University degree attribute may contain B.Eng., M.Eng., or Ph.D. Composite attributes b/w ( ) and separating with commas and multivalued b/w { } . Such attributes r called COMPOSITE ATTRIBUTES Stored versus derived Most attribute values are stored eg birthdate. BUT Derived from stored value. E.g., age from birth date Null : An attribute takes a null value when an entity does not have a value for it. eg house_no Height of a person attribute is missing n homephone attribute is not known attribute.
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