We have come across the NOT NULL constraint already in our Oracle tutorials. The NOT NULL constraint requires that the column contains a value when it is initially inserted into the table or whenever the column is updated.
Lets look at the structure of the students table.
If we attempt to update the students table and set the gender to be null we will receive the following error. Our view tells us that there are two NOT NULL constraints on the enrolments table. An Oracle table can have one and only one primary key. A primary key allows only one row to exist in the table with the particular value in the column. We can create a primary key for a table when we create the table or it can be added to the table after creation by using the alter table statement.