In Oracle, PRIMARY KEY constraint is used to ensure that all values in a column are unique and cannot contain NULL values. The PRIMARY KEY constraint forces the column to always accept a UNIQUE value without NULL. In Oracle, you can have only one PRIMARY KEY constraints on each table. The PRIMARY KEY constraint can be create on single column or group of multiple columns.
Note:
In the above example, we are creating new table called "tblCustomer" using PRIMARY KEY constraint. The PRIMARY KEY constraint enforces column "custId" to always accept UNIQUE value without NULL.