Does the concept of candidate key exist only in theory?

I know the concept of candidate key in RDBMS theory, but do candidate keys really exist in actual SQL engines? I mean is there any way to designate a particular column or set of columns as a candidate key in any of the SQL database management systems, say SQL Server, Postgres, MySQL, Oracle etc.? Is there any reserved keyword for designating column(s) as a candidate key like PRIMARY KEY or UNIQUE in case of primary key column and unique column? I feel UNIQUE constraint itself provides implementation of the candidate key concept. I don't see any practical value of having a separate CANDIDATE KEY keyword. Is it so?