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Page 4 of 17 Database ModelsA model provides a formal method of representing data in a conceptual form and provides the necessary means of manipulating the data held within the database. Most database systems are built around one particular data model, although it is increasingly common for products to offer support for more than one model. For any one logical model various physical implementations may be possible, and most products will offer the user some level of control in tuning the physical implementation, since the choices that are made have a significant effect on performance. Relational data Model
The relational model was introduced by E. F. Codd in 1970 as a way to make database management systems more independent of any particular application. It is a mathematical model defined in terms of predicate logic and set theory.
A relational data model uses attributes (columns) and tuples (rows) to contain and organize data. The relational model is the most widely used. It presents information in the form of two dimensional tables. Each table contains unique rows, columns, and cells. A primary key is a field that links all the data within a record to a corresponding value. Following are important properties of relational tables: - All values are atomic.
- Each row is unique, identified by its primary key value.
- Column values are of the same kind.
- The sequence of columns is insignificant.
- The sequence of rows is insignificant.
- Each column has a unique name.
Hierarchical data model A hierarchical data model combines records and fields that are related in a logical tree structure. The tree structure contains branches, and each branch has a number of leaves, or data fields. These databases have well-defined, pre specified access paths, but they are not as flexible in relating information as a relational database. Hierarchical databases are useful for mapping one-to-many relationships. One limitation of the hierarchical model is its inability to efficiently represent redundancy in data. LDAP uses hierarchical data model. Distributed data model A distributed data model has data stored in more than one database, but it is logically connected. Different databases may live at different places on a network or reside in totally different networks. Because they are logically connected, the users see the database as one full entity, instead of several entities. When users access data, they do not need to know which database they are accessing; one database may transfer a user to another database, which is transparent to the user.
This model enables different databases to be managed by different administrators, although one person or group must manage the entire logical database. This may be useful if each database contains information that is specialized, but the complexity of load balancing, fault tolerance, and shifting of users is quite high. Object-oriented database An object-oriented database is designed to handle a variety of data (images, audio, documents, video). An object-oriented database is more dynamic in nature than a relational database, because objects can be created when needed and the data and procedure go with the object when it is requested.
OODB have the characteristics of ease of reusing code and analysis, reduced maintenance and an easier transition from analysis of the problem to design and implementation. Its main disadvantages are a steep learning curve and high overhead of hardware and software required for development and operation.
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