UNIQUE
Remarks
To specify that the index not allow duplicate values, include the UNIQUE keyword. If the column or columns that make up the index contains duplicate values when you execute the CREATE INDEX statement with the UNIQUE keyword, PSQL returns Status Code 5 and does not create the index.
Note You should not include the UNIQUE keyword in the list of index attributes following the column name you specify; the preferred syntax is CREATE UNIQUE INDEX.
See Also