Is it permissible for one of the columns in a composite unique key to contain a NULL value?

In most relational databases, including SQL, unique keys (including composite keys) do not allow NULL values in columns that are part of the unique key. This is because NULL is considered an unknown value, and comparing NULL with NULL does not yield a true result, which violates the logic of uniqueness.

However, behavior may vary depending on the DBMS:

• In some DBMSs (e.g., PostgreSQL), unique constraints allow multiple rows with NULL in unique columns, since NULL is not considered equal to NULL.
• In others (e.g., SQL Server), unique keys do not allow NULL.

If a column that can accept NULL is needed in a composite unique key, consider using other mechanisms, such as triggers or checks, or replacing NULL with a special value.

In summary: usually, columns with NULL are not allowed in a composite unique key to ensure proper enforcement of uniqueness.