sql - Difference between a theta join, equijoin and natural join

Question

I'm having trouble understanding relational algebra when it comes to theta joins, equijoins and natural joins. Could someone please help me better understand it? If I use the = sign on a theta join is it exactly the same as just using a natural join?

Answer 1

A theta join allows for arbitrary comparison relationships (such as ≥).

An equijoin is a theta join using the equality operator.

A natural join is an equijoin on attributes that have the same name in each relationship.

Additionally, a natural join removes the duplicate columns involved in the equality comparison so only 1 of each compared column remains; in rough relational algebraic terms: ? = πR,S-as ○ ?aR=aS