A primary key constraint defines one or more columns whose values
    make a row in a table different from all others. SQL requires
    that values in a primary key column be unique and not null;
    therefore, you need not specify the UNIQUE and NOT NULL column
    constraints for primary key columns.

    You cannot specify the primary key constraint for a computed
    column.

    When used as a table constraint this clause must be followed by
    a list of column names. When used as a column constraint this
    clause applies to the named column of the table.