D The Physical Database The lowest level of the database and the only level

D the physical database the lowest level of the

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D. The Physical Database - The lowest level of the database and the only level that exists in PHYSICAL FORM. 1. Data Structures - are the bricks and mortar of the database. The data structure allows records to be located, stored and retrieved and enables movement from one record to another. 2 Fundamental Components: iv..a. Data Organization – refers to the way records are physically arranged on the secondary storage device. May be either SEQUENTIAL (stored in contiguous locations that occupy a specified area of disk space) or RANDOM (stored with regard to their physical relationship to other records of the same file). iv..b. Data Access Methods – the technique used to locate records and to navigate through the database. DBMS Models - Data Model - is an abstract representation of the data about entities, including resources (assets), events (transactions), and agents (personnel or customers, etc.) and their relationships in an organization. Purpose : to represent entity attributes in a way that is understandable to users.
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Database Terminologies: 1. Data Attribute/Field - a single item of data, such as customer’s name, account balance, or address. 2. Entity - is a database representation of an individual resource, event or agent about which we choose to collect data. 3. Record Type (Table or File) - group of data attributes that logically define an entity. 4. Database - set of record types that an organization needs to support its business processes. 5. Associations – record types that constitute a database exist in relation to other record types. 3 Basic Associations: one-to-one, one-to-many, and many-to-many. 1. The Hierarchical Model - The model from which the earliest databases are based. - The most popular method of data representation because it reflected, more or less faithfully, MANY aspects of an org that are hierarchical in relationship. - This model is called a NAVIGATIONAL DATABASE because traversing the files requires following a predefined path. - [A parent record can have one or more child records but NO child can have more than one parent.] - 2. The Network Model - Developed by Committee on Development of Applied Symbolic Languages (CODASYL) created by ANSI in the late 1970s. - The MOST POPULAR network model is the IDMS (Integrated Database Mgt System). - ALSO a NAVIGATIONAL database with explicit linkages between records and files. - [Permits a child to have multiple parents] 3. The Relational Model - Originally proposed by E.F. Codd. - Its difference for navigational database is the way in which data associations are represented to the user. - Portrays data in the form of TWO-DIMENSIONAL TABLES. Databases in a Distributed Environment 1. Centralized Databases – retaining the data in a central location. Objective: to maintain data currency. 2. Distributed Databases – can be either: - Partitioned (splits the central database into segments or partitions that are distributed to their primary users) or - Replicated (effective in companies where there exists a high degree of sharing but no primary user; data is replicated at each IT unit = data traffic between sites is reduced considerable.)
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*Deadlock Phenomenon – multiple sites lock out each other from the database, thus preventing each from processing its transactions.
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