UNIQUE Definition (unique_definition) 

A UNIQUE definition (unique_definition) in the CREATE TABLE statement defines the uniqueness of column value combinations.

Syntax

<unique_definition> ::= [CONSTRAINT <index_name>] UNIQUE (<column_name>,...)

Examples

SQL Tutorial, Indexes

Explanation

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.