While the relational model is claimed to be based on relations, the vast majority of the "relations" used in practice are functions, not general relations. A g

While the relational model is claimed to be based on relations, the vast majority of the "relations" used in practice are functions, not general relations. A general relation exists only between the columns of a table that are included in a multi-column primary key. All columns that are not part of the primary key are functions of the primary key. Most tables used in practice use a single column as the primary key, which is frequently just a number or a UUID. Most databases contain only tables that are functions, without any table that contains general relations. The most frequently used kinds of joins are just function compositions.