Sets, relations and functions

In June 1970 Edgar F. Codd, a mathematician at IBM's San Jose research laboratory, published “A Relational Model of Data for Large Shared Data Banks”. His proposal was to stop thinking of data as records linked by pointers and to treat a table as a relation in the mathematical sense: a set of tuples.

That one decision is why SQL has UNION, INTERSECT and EXCEPT, why a table has no meaningful row order and why a primary key is a statement about a function. The mathematics came from set theory, which Georg Cantor had begun developing in the 1870s.