alt Hacker NewsWhile 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.
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.