Question

How can the NOT NULL constraint be added to a column after a table is made?

a) By using the MODIFY COLUMN statement in SQL
b) It cannot be added after table creation
c) By using the RENAME COLUMN statement in SQL
d) By altering the column using the ADD CONSTRAINT statement

Answer 1

To add a NOT NULL constraint to an existing column, use the ALTER TABLE statement with MODIFY COLUMN command in SQL and ensure no NULL values are present.

Step-by-step explanation:

The NOT NULL constraint can be added to an existing column in a table after the table is created by using the ALTER TABLE statement along with the MODIFY COLUMN command in SQL. This modification ensures that the column cannot have NULL values in the future. However, before adding the NOT NULL constraint, you must ensure that all existing rows in the table have non-NULL values for that column, as it's not possible to enforce the NOT NULL constraint on a column with existing NULL values.

The MODIFY COLUMN statement mentioned in option a) is not a valid SQL statement. Option b) is incorrect, as the NOT NULL constraint can be added after table creation. The RENAME COLUMN statement in option c) is used to rename a column, not to add a constraint