relaatiomallista

Relatiomallista, also known as relational algebra, is a formal system used in relational database theory for defining data and manipulating data stored in relational databases. It was introduced by Edgar F. Codd, the father of relational databases, in 1970. Relational algebra consists of a set of operations that take one or more relations as input and produce a new relation as output. These operations are typically classified into two categories: unary operations, which take a single relation as input, and binary operations, which take two relations as input.

The basic operations in relational algebra include selection, projection, union, intersection, difference, Cartesian product, and join. Selection, also known as restrict, is an operation that selects tuples from a relation that satisfy a given predicate. Projection is an operation that selects a subset of attributes from a relation. Union, intersection, and difference are set operations that combine or compare two relations. Cartesian product is an operation that combines two relations by pairing each tuple from the first relation with each tuple from the second relation. Join is a binary operation that combines two relations based on a common attribute.

Relational algebra provides a powerful and expressive language for querying and manipulating relational databases. It is the basis for the SQL query language, which is widely used in practice. Relational algebra is also used in the design and optimization of relational databases, as well as in the study of database theory.