Two-Column UNIQUE - Unique Constraints Step by Step

Composite UNIQUE checks the whole key tuple.

highlighted = computed this step

Composite key

A two-column UNIQUE rule checks the whole key tuple.

\text{two-column key}

Read tuple keys

The rule uses 2 key columns and finds 1 composite violation.

\text{key columns}=2

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.

Both values must match

Rows collide only when every non-NULL component in the tuple matches.

\text{whole tuple matches}

Summary

Composite UNIQUE compares tuples, not columns one by one in isolation.

\text{tuple rule}