In SQL, the NOT NULL constraint is used to specify that a column must always contain a value and cannot be left empty (NULL). It ensures that every row in the table has a non-null value in the specified column. The NOT NULL constraint can be applied during table creation or modified later using the ALTER TABLE statement. Here's an example of how to use the NOT NULL constraint:

To apply the NOT NULL constraint during table creation:

sql
CREATE TABLE table_name (
    column_name data_type NOT NULL,
    ...
);

To add the NOT NULL constraint to an existing column using ALTER TABLE:

sql
ALTER TABLE table_name
    ALTER COLUMN column_name SET NOT NULL;

To drop the NOT NULL constraint from a column using ALTER TABLE:

sql
ALTER TABLE table_name
    ALTER COLUMN column_name DROP NOT NULL;

Replace `table_name` with the name of the table, and `column_name` with the name of the column you want to apply or modify the NOT NULL constraint on.

It's important to note that applying the NOT NULL constraint to a column may require existing rows to have a valid non-null value in that column. If any rows violate the constraint, you need to update the existing data to meet the constraint before applying the NOT NULL constraint.

The NOT NULL constraint helps enforce data integrity by ensuring that required columns always have a value, and it helps prevent null-related errors in queries and operations on the table.