Relational Algebra
A set-theoretic abstract model:
- An N-ary Relation is a Set of relation instances.
- N is called the ‘arity’ or dimension of the relation.
Each N-ary relation instance is an N-tuple (N-dimensional vector) which groups, associates, or relates one specific instance from each of the relation’s N component data types or classes.
- Example: Ternary relation R(x,y,z) relates elements of entity types X, Y, Z so it is a set of 3-tuples: {<x,y,z>}
An algebraic model, with operations defined among its relation members:
- union, intersection, projection, (subset) selection;
- ‘joins’ of various types.
Questions: What is ‘join’? What is ‘projection’?