Relation

A relation is a set of tuples over a fixed set of attributes in the relational model. It is commonly represented as a table, with tuples as rows and attributes as columns.

Tuple: An ordered assignment of values to named attributes, representing one record in a relation. Each row of a relation corresponds to one tuple.
Relational algebra: A formal system of operations for retrieving and transforming relations. Its operators define how query results are formed from relations.
SQL table: A tabular database structure defined by SQL, which may allow duplicate rows and null values. SQL tables resemble relations but need not satisfy the classical set-based definition.
Relational database: A database that organizes data into relations and manipulates them using relational principles. Relational databases store information in structures modeled on relations.
Edgar F. Codd: A British computer scientist who formulated the relational model for databases. Codd introduced relations as the foundation for a new database model.
Attribute: A named component of a relation whose values come from a specified domain. Attributes define the named positions shared by every tuple.
Selection (relational algebra): An operation that returns tuples satisfying a specified predicate. Selection filters a relation's tuples without changing its attributes.
Bag (mathematics): A collection in which elements may occur more than once and each occurrence has a count. A bag permits duplicate tuples, unlike a relation's set semantics.
SQL: A declarative language for defining, querying, and modifying data in relational databases. SQL exposes relation-like tables through data definition and query statements.
A Relational Model of Data for Large Shared Data Banks: Edgar F. Codd's 1970 paper proposing a relational model for managing shared databases. This paper established the model in which relations became central data structures.

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