Relational Algebra
JOIN
Returns a relation containing all possible tuples that are a combination of two tuples, one from each of two specified relations, such that the two tuples contributing to any given result tuple have common values for the common attributes of the two relations (and those common values appear just once, not twice, in that result tuple). Note: This kind of join was originally called the natural join, to distinguish it from various other kinds—especially the so called θ-join (“theta join”).