Once you specify the primary key for each table, no further action is needed by client application developers or by the database administrator to maintain entity integrity.
The table owner defines the primary key for a table when creating it. If the structure of a table is modified at a later date, the primary key may also be redefined using the ALTER TABLE statement clauses DELETE PRIMARY KEY or ADD PRIMARY KEY. See Reference: Statements and Options.
Some application development systems and database design tools allow you to create and alter database tables. If you are using such a system, you may not have to enter the CREATE TABLE or ALTER TABLE command explicitly: the application generates the statement itself from the information you provide. See Reference: Statements and Options.