1  –  COMMENT_IS

    Adds a comment about the constraint. SQL displays the text of the
    comment when it executes a SHOW CONSTRAINTS statement. Enclose
    the comment in single quotation marks ( ') and separate multiple
    lines in a comment with a slash mark (/).

2  –  constraint-attributes

    See the ALTER TABLE statement.

3  –  constraint-name

    The name of the table whose definition you want to change.

4  –  RENAME_TO

    Changes the name of the constraint being altered. See the the
    RENAME statement for further discussion. If the new name is the
    name of a synonym then an error will be raised.

    The RENAME TO clause requires synonyms be enabled for this
    database. Refer to the ALTER DATABASE statement SYNONYMS ARE
    ENABLED clause. Note that these synonyms may be deleted if they
    are no longer used by database definitions or applications.