Postgresql: Conditionally unique constraint

postgresql constraints unique-constraint
EoghanM picture EoghanM · Apr 26, 2013 · Viewed 42k times · Source

I'd like to add a constraint which enforces uniqueness on a column only in a portion of a table.

ALTER TABLE stop ADD CONSTRAINT myc UNIQUE (col_a) WHERE (col_b is null);

The WHERE part above is wishful thinking.

Any way of doing this? Or should I go back to the relational drawing board?

Answer

Craig Ringer picture Craig Ringer · Apr 26, 2013

PostgreSQL doesn't define a partial (i.e. conditional) UNIQUE constraint - however, you can create a partial unique index. PostgreSQL uses unique indexes to implement unique constraints, so the effect is the same, you just won't see the constraint listed in information_schema.

CREATE UNIQUE INDEX stop_myc ON stop (col_a) WHERE (col_b is NOT null);

See partial indexes.

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