Monday, April 19, 2010

data base management system

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A database consists of a number of tables. Each table comprises of rows(records) and columns(attributes). Each record contains values for the corresponding attributes. The values of the attributes for a record are interrelated. For example, different cars have different values for the same specifications (length, color, engine capacity, etc.).
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In the database oriented approach, we store the common data in one table and access it from the required tables. Thus the redundancy of data decreases.
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The database oriented approach supports multiple views of the same data. For example, a clerk may only be able to see his details, whereas the manager can view the details of all the clerks working under him.
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Multiple views of the same database may exist for different users. This is defined in the view level of abstraction.
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The logical level of abstraction defines the type of data that is stored in the database and the relationship between them.
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The design of the database is known as the database schema.
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The instance of the database is the data contained by it at that particular moment.
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The Database Administrator has the total control of the database and is responsible for the setting up and maintaining the database.

Languages :
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The DDL (Data Definition Language) is used to define the schema of the database and relations between entities.
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The DML (Data Manipulation Language) enables us to access and operate upon the data in the database.
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The DCL (Data Control Language) is used to control the access to the database.
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The Procedural DML requires the user to state the information required and how to get it whereas the Non-Procedural DML does not require the user to specify how to get the information.
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The query processor breaks down the DML statements into statements that can be comprehended by the DBMS.
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