A UNIQUE definition (unique_definition) in the CREATE TABLE statement defines the uniqueness of column value combinations.
SQL Tutorial, Indexes
Specifying a UNIQUE definition in a CREATE TABLE statement has the same effect as specifying the CREATE TABLE statement without a UNIQUE definition, but with a CREATE INDEX statement with UNIQUE.
· Index name specified: The generated index is stored under this name in the database catalog.
· No index name: The database system assigns a unique index name to the index.