UNIQUE Definition (unique_definition)

Use

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

Structure

				
<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 index generated is stored under this name in the database catalog.
No index name: The database system assigns a unique index name to the index.