A one-column UNIQUE rule checks one key cell per row.
one key cell
Read email keys
The rule uses 1 key column and finds 1 violation.
violations=1
UNIQUE constraint examples are tiny finite table checks; SQL dialect completeness, index implementation, locking, optimizer behavior, product behavior, and performance claims are out of scope.
Key rows are visible
The key table shows the extracted email key for every source row.
key per row
Summary
One-column UNIQUE compares one displayed key at a time.