CREATE INDEX is a Teradata extension to the ANSI SQL:2011 standard. The ANSI SQL standard does not define DDL for creating indexes.
The CREATE UNIQUE INDEX syntax is functionally equivalent to adding a UNIQUE NOT NULL or PRIMARY KEY NOT NULL constraint set to a column in ANSI SQL:2011 if all of the columns in the column set are NOT NULL.
A CREATE UNIQUE INDEX statement on a column set where one or more columns are nullable is similar.