Relational Algebra

Generally, the result of a relational algebra operation is a new relation.

Unary operations

Unary operations include selection and projection.

• Selection

Selection could return a new relation consisting of certain rows of the selected relation.

Predicate F is a logical expression, indicating the criteria of selection.

Selection

F = X θ Y, with
X and Y being attributes, constants, simple functions, and
θ standing for relational operators >, >=, <, <=, =, or !=.

Also, F could use logical operators AND, OR, NOT to connect sub-predicates.

• Projection

Projection could return columns satisfying predicate criteria F.

Projection

Binary operations

• Join

Join operation selects rows from Cartesian product of two relations.

Theta join:
Join two relations under predicate F (with θ operation).
Or say,
select rows from Cartesian product with predicate F.

Equi-join:
Theta join with the θ being "=".
Two relations have same values on certain columns.

Natural join:
Special Equi-join.
Two relations have same attribute and join together after eliminate one duplicate column.