A keyword that specifies whether or not each value of the index
    must be unique. If you try to store the same value twice in a
    column or set of columns that have an index defined as UNIQUE,
    SQL returns an error message the second time and does not store
    or modify the row that contains the value. This is true for null
    values as well as any other value.

    If you specify UNIQUE, SQL checks as it executes the CREATE INDEX
    statement to see if the table already contains duplicate values
    for the index.