Joins-Introduction
Joins
The join operation is one of the most useful operations in relational algebra and is the most commonly used way to combine information from two or more relations. Although a join can be de ned as a cross-product followed by selections and projections, joins arise much more frequently in practice than plain cross-products. Further, the result of a cross-product is typically much larger than the result of a join, and it
is very important to recognize joins and implement them without materializing the underlying cross-product (by applying the selections and projections `on-the-fly'). For these reasons, joins have received a lot of attention, and there are several variants of the join operation.
