Can a value in a column with a FOREIGN KEY constraint be NULL?

Yes, possibly, if the conditions are met:

• The column with the FOREIGN KEY constraint does not have a NOT NULL constraint.
• The reference is made to a column that allows NULL values (usually a primary key or a unique key).

If the value in the FOREIGN KEY column is NULL, it means that the row in the dependent table is not linked to any row in the parent table.