How to make a NULLable column to NOT NULL in oracle

Question

How to make a NULLable column (varchar2(20)) to NOT NULL of a table in oracle where I have more than 1 million records in the table

Answer 1

NOT NULL constraint specifies that a column cannot contain NULL values. To add a NOT NULL constraint to an existing table by using the ALTER TABLE statement.

ALTER TABLE table_name MODIFY ( column_name NOT NULL);

In this case, the column_name must not contain any NULL value before applying the NOT NULL constraint.