Is there a sql database engine that support "specific value duplicate restriction"? For example, in the brand column of a product database, the brand value "unknown" should only exist once while other brand values can exist many times.

Is there a sql database engine that supports it? or should I do it in application layer?
Some DBMSes support indexes with a WHERE-condition

CREATE unique INDEX OnlyOneUnkownBrand
ON product (brand)
WHERE brand = 'unknown';