Difference between a theta join, equijoin and natural join

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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

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