Group of characters used to identify a particular database element is known as
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Chapter 6
Database Management
6.1 Hierarchy of Data [Figure 6.1][Slide 6-4]
Data are the principal resources of an organization. Data stored in computer systems form a hierarchy extending from a single bit to a database, the major record-keeping entity of a firm. Each higher rung of this hierarchy is organized from the components below it.
Data are logically organized into:
1. Bits (characters)
2. Fields
3. Records
4. Files
5. Databases
Bit (Character) - a bit is the smallest unit of data representation (value of a bit may be a 0 or 1). Eight bits make a byte which can represent a character or a special symbol in a character code.
Field - a field consists of a grouping of characters. A data field represents an attribute (a characteristic or quality) of some entity (object, person, place, or event).
Record - a record represents a collection of attributes that describe a real-world entity. A record consists of fields, with each field describing an attribute of the entity.
File - a group of related records. Files are frequently classified by the application for which they are primarily used (employee file). A primary key in a file is the field (or fields) whose value identifies a record among others in a data file.
Database - is an integrated collection of logically related records or files. A database consolidates records previously stored in separate files into a common pool of data records that provides data for many applications. The data is managed by systems software called database management systems (DBMS). The data stored in a database is independent of the application programs using it and of the types of secondary storage devices on which it is stored.
6.2 File Environment and its Limitations
There are three principal methods of organizing files, of which only two provide the direct access necessary in on-line systems.
File Organization [Figure 6.2 & 6.3]
Data files are organized so as to facilitate access to records and to ensure their efficient storage. A tradeoff between these two requirements generally exists: if rapid access is required, more storage is required to make it possible.
Access to a record for reading it is the essential operation on data. There are two types of access:
1. Sequential access - is performed when records are accessed in the order they are stored. Sequential access is the main access mode only in batch systems, where files are used and updated at regular intervals.
2. Direct access - on-line processing requires direct access, whereby a record can be accessed without accessing the records between it and the beginning of the file. The primary key serves to identify the needed record.
There are three methods of file organization: [Table 6.1]
1. Sequential organization
2. Indexed-sequential organization
3. Direct organization
Sequential Organization
In sequential organization records are physically stored in a specified order according to a key field in each record.
Advantages of sequential access:
1. It is fast and efficient when dealing with large volumes of data that need to be processed periodically (batch system).
Disadvantages of sequential access:
1. Requires that all new transactions be sorted into the proper sequence for sequential access processing.
2. Locating, storing, modifying, deleting, or adding records in the file requires rearranging the file.
3. This method is too slow to handle applications requiring immediate updating or responses.
2. It makes it possible to create, access, maintain, and control databases.
3. Through a DBMS, data can be integrated and presented on demand.
Advantages of a database management approach:
1. Avoiding uncontrolled data redundancy and preventing inconsistency
2. Program-data independence
3. Flexible access to shared data
4. Advantages of centralized control of data
6.4 Levels of Data Definition in Databases
1. Schema - is an overall logical view of the relationships between data in a database.
2.Subschema - is a logical view of data relationships needed to support specific end user application programs that will access the database.
3.Physical - looks at how data is physically arranged, stored, and accessed on the magnetic disks and other secondary storage devices of a computer system.