Finding Orphaned Records — Checking Referential Integrity Without a Foreign Key
A foreign key pointing into the void is quick to describe — a country_code with no matching entry in the reference data — and surprisingly easy to check incorrectly. The most intuitive phrasing is, of all things, the most dangerous: NOT IN (SELECT …) reads like plain English but silently collapses the moment the reference column holds a single NULL. The … Read more