A primary key in ORACLE implies unique and not null, implies u cannot leave the column empty during an insert. In mysql, a column defined as a primary key accepts if nothing is inserting into that column during an insert, for the first time. The next time the same is performed(ie., leaving the primary key column empty), it is violation of primary key constraint.
That is, mysql's definition of primary key implies only unique but not NOT NULL. Wat do i do to have a column both unique and notnull?