In addition to the data itself, a set of program is necessary to facilitate adding new data as well as modifying and retrieving existing data within a database. This set of programs is referred to as database management system (DBMS). Generally, the user of a database management system accesses the database via a special query language or via applications programs written in a high-level language. The applications program utilizes special host- or command-language instructions to communicate requests for data to the portion of the DBMS referred to as the database processing system. Basically, the database processing system consists of a series of programs that translate requests from users or user programs into the instructions necessary to access, add to, change, or delete records or items of data from database. Thus, an applications program need include only a command to access the desired data element. The actual mechanics of locating and accessing the data element are accomplished by the database processing programs. Accessing Data from a database in response to an applications program request. - Application program instruction initiates a request for data
- Control unit transfers control to DBMS
- DBMS verifies that the request is valid and determines location of data element in database
- DBMS issues command to access data from secondary storage
- Data element is accessed and stored in a buffer of the DBMS
- DBMS transfers data element to application program storage area
- Next instruction in application program is accessed and executed
Database - Advantages & Disadvantages
Advantages - Reduced data redundancy
- Reduced updating errors and increased consistency
- Greater data integrity and independence from applications programs
- Improved data access to users through use of host and query languages
- Improved data security
- Reduced data entry, storage, and retrieval costs
- Facilitated development of new applications program
Disadvantages
- Database systems are complex, difficult, and time-consuming to design
- Substantial hardware and software start-up costs
- Damage to database affects virtually all applications programs
- Extensive conversion costs in moving form a file-based system to a database system
- Initial training required for all programmers and users
Query Language
In addition to a data dictionary, most DBMSs also provide a query language that enables a user to request and receive information directly from the database system. The complexity of this query language and the level of difficulty using it varies greatly with the DBMS. Some query languages are procedural and require step-by-step instructions to be written, as with a high-level language. Others accept English-like commands or guide the user with a series of fill-in questions, or even display menu on the terminal monitor form which the user must select a desired command. Query languages supplied with microcomputer database packages are almost always used to interact with the database and provide built-in help for novice users. |
Data Structures
The structures of the data within a database can be viewed in two ways, physically and logically. The physical data structure refers to the physical arrangement of the data on the secondary storage device, usually disk. Typically, this physical structure is the concern of specialists who design DBMSs. Analysts, programmers, and users are generally less concerned with the physical structure than with the logical structure. The logical data structure concerns how the data "seem" to be arranged and the meanings of the data elements in relation to one another. This structure, or model, is generally defined in terms of a schema- an overall conceptual view of the logical relationships between the data elements in the database. It includes the names of the major elements, their attribute, and the logical relationships between them. The figure below shows a schema that might apply to database of an autoparts store. Although greatly simplified, it illustrates that relationships exist between part numbers, types of cars, models, and manufactures. The connecting lines indicate links and the arrows the type of relationship. For example, in this schema, manufacturer is linked to part number, and the relationship in one-to-many (a double-headed arrow implies many and single-headed arrow one). That is, one manufacturer produces many part numbers, but each part number has only one manufacturer. There are also indirect links indicated. A part number, for example, is linked to type of vehicle through model. |
Logical Data
Logical Data Structures
Several logical data structures are used to express the relationships between individual data elements or records in a database. Common logical data structures are hierarchical, network, and relational, with relational being predominant.
Hierarchical Structure
In a hierarchical structure sometimes referred to as a tree structure, the stored data get more and more detailed as on branches further and further out on the tree. Each segment, or node, may be subdivided into two or more subordinate nodes, which can be further subdivided into two or more additional nodes. However, each node can emanate form only one "parent". To the user, each record resembles an organizational chart in which the segments or nodes fit into a well-defined hierarchy or tree. There is only one segment, or root, at the top. Applications programs process hierarchical databases one record at a time, as with conventional file structures. Hierarchical database structures are commonly used with large mainframe computer systems.
Network Structure
The network structure is similar to the hierarchical structure with the exception that in the network structure, a node may have more than one parent. The trade-off between the simplicity of design of a hierarchical structure and the storage efficiency of a network structure is a very important consideration in database implementation. Network structures are most commonly used with mainframe and minicomputer systems, rarely with microcomputers.
Relational Structure
The relational structure organizes data in two-dimensional tables. These tables offer great flexibility and a high degree of data security. The relational structure uses relatively little memory or secondary storage. Unfortunately, the process of creating these tables is rather elaborate. Another disadvantage of this structure is that it generally requires more time to access information than does either of the other two structures. This is because much more information must be searched in order to answer queries posed to the system. In addition, some implementations use a fixed amount of storage for each field, resulting in inefficient storage utilization. In spite of these disadvantages, the relational structure has gained rapid acceptance and is currently the most popular of the three structures. This structure is used almost exclusively with microcomputer systems and is increasingly being applied to minicomputer and mainframe systems. Many experts predict that it will eventually replace the other structures completely.
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http://en.wikipedia.org/wiki/Database |
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