Database management system (DBMS) is a computer software program that manages a database so that information on the database can be organized, retrieved, searched, and changed (McCain & Merril, 2001).
References
McCain, M. M. & Merril, M. (2001). Dictionary for school library media specialists: A practical and comprehensive guide. Greenwood Village, CO: Libraries Unlimited. A DBMS is a set of programs used to define, administer, and process databases and their associated applications. The database being managed, in essence, a structure that is built to hold valuable data. A DBMS is the tool that is used to build that structure and operate on the data contained within the database. Whatever the size of the computer that hosts the database – and regardless of whether the machine is connected to a network – the flow of information between database and user is always the same. The DBMS masks the physical details of the database storage so that the application only has to concern itself with the logical characteristics of the data, not with how the data is stored.
Two key parts of a database management system are (1) the user defined data or data tables and (2) the operations defined to manipulate the data. The data tables, often called entities or touples, contain the database information. The tables are composed of constituent characteristics, or attributes. The DBMS provides all the basic service required to organize and maintain the database, including the following:
· Moving the data to and from the physical data files as needed
· Managing concurrent data access by multiple users, including provisions to prevent simultaneous updates from conflicting with one another
· Managing transactions so that each transaction’s database changes are an all-or-nothing unit of work. In other words, if the transaction succeeds, all database changes made by it are recorded in the database; if the transaction fails, none of the changes it made are recorded in the database
· Support for a query language, which is a system of commands that a database user employs to retrieve data from the database
· Provisions for backing up the database and recovering from failures
· Security mechanisms to prevent unauthorized data access and modification
Layers of Data Abstraction
Databases have the unique capability of presenting multiple users of the data with their own distinct views of that data while storing the underlying data only once. These are collectively called user views. A user in this context is any person or application that signs on to the database for the purpose of storing and/or retrieving data. Below are the layers of data abstraction:
1. Physical Layer – the physical layer contains the data files that hold all the data for the database. Nearly all modern DBMSs allow the database to be stored in multiple data files, which are usually spread out over multiple physical disk drives. With this arrangement, the disk drives can work in parallel for maximum performance.
2. Logical Layer – the logical layer of logical model is the first of two layers of abstraction in the database. Physical layer has a concrete existence in the operating system files, whereas logical layer exists only as abstract data structures assembled from the physical layer as needed. The DBMS transforms the data in the data files into a common structure. This layer is sometimes called the schema, a term used for the collection of all data items stored in a particular database. Depending on the particular DBMS, this can be a set of two-dimensional tables, a hierarchical structure similar to a company’s organization chart, or some other structure.
3. External Layer – the external layer is the second layer of abstraction in the database. This layer is composed of the user views discussed earlier, which are collectively called the subschema. This is the layer where users and application programs that access the database connect and issue queries against the database.
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