<key_definition> ::= PRIMARY KEY (<column_name>,...)
SQL Tutorial, Primary Key
The column name column_name must identify a column in the base table. The specified column names are key columns in the table.
A key column must not identify a column of the data type LONG and is always a NOT NULL column. The database system ensures that no key column has a NULL value and that no two rows of the table have the same values in all key columns.