Relational data model

The relational model is a data model with a thorough mathematical foundation. An attribute is a named set of atomic values, the domain. A relation table is a subset of the cross-product of the relation attributes.

The relational data model separates the logical data definition from physical storage.

Two data manipulation languages have been developed for the relational data model:

• Relational algebra is a procedural language which consists of the set operators union, intersection, difference and the relational operators selection, projection, and join. The selection operator selects rows from a relation table that meet a selection condition. The projection operator selects specific attributes from a relation table. The join operator merges relation tables according to a comparison condition over attributes from the original relation tables.

  The result of a relational operation is again a relation, so that the operators can be nested.

• Relational calculus is a declarative language based on first-order predicate logic.

  An expression is a term of the form with variables over a domain and COND a formula that is either true or false. The variable bindings on the left side of the vertical bar are the values returned as the result of a query, and COND consists of atomic formulas which are connected through logical AND and OR operators.

The relational data model makes no assumptions about the relationships between entities. In general, an entity is mapped to a relation table, with the properties of an entity described through the relation attributes. Meaningful relationships between entities are expressed through relational algebra operations or calculus expressions. Hence, complex relationships between entities can be expressed, with the notable expression of recursive relationships.