In relational algebra, the natural join of two tables can be described as:

A natural join uses common attribute names automatically. In relational algebra it can be written as projection over a selection on the Cartesian product.

Formal idea: R ⋈ S = π(attributes of R ∪ attributes of S)(σ(common attributes equal)(R × S))

1. Cartesian product: start with R × S.

2. Filtering/selection: keep only tuple pairs where every common attribute has equal values. This is the filtered Cartesian product.

3. Projection: remove duplicate copies of the common attributes and keep the final joined attributes.

Correct answer: Combination of Projection and filtered Cartesian Product.

Why not selection and filtered Cartesian product? Because “filtered Cartesian product” already includes the selection/filtering idea, while natural join still needs projection.