[adrian_b]

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.

[thesz]

While it is true on the schema level, it is not true at the query level.

The most frequently used kind of join is a general relation.

[1] https://github.com/agirish/tpcds/blob/master/query1.sql

Query 1 from TPC-DS creates a multi-column relation by using GROUP BY. Which relation is then partially constrained by different columns from different tables.