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Relational database management systems (RDBMS) support the relational (=table-oriented) data model. The schema of a table (=relation schema) is defined by the table name and a fixed number of attributes with fixed data types. A record (=entity) corresponds to a row in the table and consists of the values of each attribute. A relation thus consists of a set of uniform records.
The table schemas are generated by normalization in the process of data modeling.
Certain basic operations are defined on the relations:
- classical set operations (union, intersection and difference)
- Selection (selection of a subset of records according to certain filter criteria for the attribute values)
- Projection (selecting a subset of attributes / columns of the table)
- Join: special conjunction of multiple tables as a combination of the Cartesian product with selection and projection.
These basic operations, as well as operations for creation, modification and deletion of table schemas, operations for controlling transactions and user management are performed by means of database languages, with SQL being a well established standard for such languages.
The first relational database management systems appeared on the market at the beginning of the 1980s and since have been the most commonly used DBMS type.
Over the years, many RDBMS have been expanded with non-relational concepts such as user-defined data types, not atomic attributes, inheritance and hierarchies, which is why they are sometimes referred to as object-relational DBMS.
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