Databases contain stored information and are structured in various ways. Legacy systems often use hierarchical or networked structures. Hierarchical databases are structured similar to an organizational chart. Lower data elements are dependent on (communicate data between) a single data element above them. Each data element can have multiple elements linked below it, but only one link to a data element above it. Networked databases are similar to hierarchical databases, but data elements can link to multiple elements above or below them.
Relational databases, which are currently the most prevalent type database, are organized as tables with data structured in rows and columns. (Data paths are not predefined because relationships are defined at the data value level.) Rows (or records) contain information that relates to a single subject, such as a customer, employee, or vendor. Columns (or fields) contain information related to each subject, such as customer identification numbers, dates of birth, or business addresses. Each record (item in a row), links to a corresponding field element (item in a column) that is defined as the primary key. Primary keys are the main identifiers associated with stored information and facilitate access to the information. In certain situations, the primary key may be comprised of data from more than one field.
Relational databases are usually comprised of multiple tables, which may reside on a single server or in a distributed environment. If related records are stored on multiple tables (for example, if a customer's primary information, as shown in the table above, is maintained at a customer service center and the customer's deposit account information is maintained on a second table/database at a local branch), the same primary key must be used in both tables to ensure data integrity. The keys in secondary tables, however, are referred to as foreign keys.
Object-relational databases have been developed that apply ad hoc object-oriented protocols in relational database environments. Definitive standards do not currently exist to support the wide acceptance of strictly object-oriented databases and database management systems. However, various proprietary standards do exist, and organizations are attempting to develop standardized object-oriented database protocols.
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