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[5] [6] A relational database definition (database schema, sometimes referred to as a relational schema) can thus be thought of as a collection of named relation schemas. [7] [8] In implementations, the domain of each attribute is effectively a data type [9] and a named relation schema is effectively a relation variable (relvar for short).
The formal definition of a database schema is a set of formulas (sentences) called integrity constraints imposed on a database. [citation needed] These integrity constraints ensure compatibility between parts of the schema. All constraints are expressible in the same language.
A relation schema (,) consists of a header and a predicate () that is defined for all relations with header . A relation satisfies a relation schema ( H , C ) {\displaystyle (H,C)} if it has header H {\displaystyle H} and satisfies C {\displaystyle C} .
The term "schema" refers to the organization of data as a blueprint of how the database is constructed (divided into database tables in the case of relational databases). The formal definition of a database schema is a set of formulas (sentences) called integrity constraints imposed on a database.
The relational algebra uses set union, set difference, and Cartesian product from set theory, and adds additional constraints to these operators to create new ones.. For set union and set difference, the two relations involved must be union-compatible—that is, the two relations must have the same set of attributes.
By this definition, RDBMS products typically implement some but not all of Codd's 12 rules. A second school of thought argues that if a database does not implement all of Codd's rules (or the current understanding on the relational model, as expressed by Christopher J. Date, Hugh Darwen and others), it is not relational. This view, shared by ...
Rather, they show entity sets (all entities of the same entity type) and relationship sets (all relationships of the same relationship type). For example, a particular song is an entity, the collection of all songs in a database is an entity set, the eaten relationship between a child and his lunch is a single relationship, and the set of all ...
Three key terms are used extensively in relational database models: relations, attributes, and domains. A relation is a table with columns and rows. A relation is a table with columns and rows. The named columns of the relation are called attributes, and the domain is the set of values the attributes are allowed to take.