When is it permissible for a primary key value to be missing or null in a database?

In relational databases, the primary key value cannot be NULL because it serves as a unique identifier for a record. However, there are situations where NULL is allowed:

• If the field is not a primary key but is indexed or used as a foreign key.
• In some NoSQL or non-relational databases, there may be different rules allowing the absence of a key.
• In composite keys, individual parts can be NULL if schema permits, but this is generally not recommended.

In classic relational DBMS, the primary key must always have a unique and non-NULL value.