Equivalent Representations of a Relation

Equivalent Representations of a Relation

Relations are sets of tuples, not lists of tuples. Therefore the order in which the tuples of a relation are presented is unimportant. For instance, we can list the three tuples of the following figure, in any of their six possible orders, and the relation is "the same" as shown in figure.

The relation Movies

Furthermore, we can reorder the attributes of the relation as we choose, without changing the relation. Nevertheless, when we reorder the relation schema, we must be careful to remember that the attributes are column headers. Therefore, when we change the order of the attributes, we also change the order of their columns. When the columns move, the components of tuples change their order as well. The result is that each tuple has its components arranged in the same way as the attributes are arranged.

For example, the following figure shows one of the many relations that could be obtained from above figure by arranging rows and columns. These two relations are considered "the same". More exactly, these two tables are different presentations of the same relation.

Another presentation of the relation Movies