Add One More Key Column - Unique Constraints Step by Step

A wider key can distinguish rows.

highlighted = computed this step

Extra key column

Adding a tie-like extra column can make formerly matching tuples different.

\text{extra key breaks tie}

Read the wider key

The wider rule uses 3 key columns and finds 0 violations.

\text{violations}=0

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.

Not a recommendation

This only shows the finite key check, not schema-design advice.

\text{mechanism only}

Summary

A wider key can distinguish rows that a narrower key grouped together.

\text{wider key}