Statement on Union Operation in Relational Algebra

Relational algebra is a set of advanced operations with relations as the operation object. Since relation is defined as a set of tuples with the same number of attributes, the operation of set algebra can be introduced into relation algebra.

The operations in relational algebra can be divided into two categories: traditional relational operations, union, difference, intersection, cartesian product (multiplication) and inverse operation of cartesian product (division); Extended relational operations, such as vertical segmentation (projection), horizontal segmentation (selection), relational combination (connection, natural connection), etc.

Traditional relational operations Traditional relational operations originated from the set theory in mathematics, and there are the following types: cartesian product operation, difference operation, intersection operation and union operation 1.2 special relational operation, and selecting projection to connect Divison. These five relational algebraic operations are: union, difference, Cartesian product, projection and selection.

In relational algebraic operations, the formula which is composed of five basic operations in finite times is called relational algebraic expression. The operation result of this expression is still a relationship. We can use relational algebraic expressions to represent various data query operations.