Data engineering·Quality, tests and knowing it is wrong
the key you have been treating as unique turns out to have four hundred duplicates, which is why the join inflated the total.
Uniqueness test
Also calledprimary key test, duplicate check
Asserting that a column or combination is unique at the table's stated grain. It is the test that most directly protects against fan traps, because a duplicated key is how a correct query returns an inflated number. It is also the cheapest way to detect an at-least-once pipeline that has stopped deduplicating, which is a failure with no other symptom.