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Thread: Rdbms notes

  1. #1

    Zip 32 Rdbms notes

    Chapter 3: Relational Model
    Structure of Relational Databases
    Relational Algebra
    Tuple Relational Calculus
    Domain Relational Calculus
    Extended Relational-Algebra-Operations
    Modification of the Database
    Example of a Relation
    Attribute Types
    Each attribute of a relation has a name
    The set of allowed values for each attribute is called the domain of the attribute
    Attribute values are (normally) required to be atomic, that is, indivisible
    E.g. multivalued attribute values are not atomic
    E.g. composite attribute values are not atomic
    The special value null is a member of every domain

    The null value causes complications in the definition of many operations
    we shall ignore the effect of null values in our main presentation and consider
    their effect later
    Relation Schema
    A1, A2, , An are attributes
    R = (A1, A2, , An ) is a relation schema
    E.g. Customer-schema =
    (customer-name, customer-street, customer-city)

    r(R) is a relation on the relation schema R
    E.g. customer (Customer-schema)
    Relation Instance
    The current values (relation instance) of a relation are specified by a table
    An element t of r is a tuple, represented by a row in a table

    n A database consists of multiple relations
    n Information about an enterprise is broken up into parts, with each relation storing one part of the information

    E.g.: account : stores information about accounts
    depositor : stores information about which customer
    owns which account
    customer : stores information about customers

    n Storing all information as a single relation such as
    bank(account-number, balance, customer-name, ..)
    results in

    H repetition of information (e.g. two customers own an account)
    H the need for null values (e.g. represent a customer without an account)

  2. #2

    Re: Rdbms notes

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  3. #3

    Re: Rdbms notes

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  4. #4

    Re: Rdbms notes

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  5. #5

    Re: Rdbms notes

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