Is it permissible to use NULL as the first key in a database?

The use of NULL as a key value in a database depends on the type of key and the specific DBMS.

• In most relational databases, the primary key cannot accept NULL values, as it must uniquely identify a record.
• A foreign key can accept NULL if the schema allows it, indicating the absence of a relationship.
• NULL values can usually be present in indexes, but in unique indexes, the behavior depends on the DBMS.

In summary: NULL is not allowed as a primary key value, but it can be used in other types of keys or indexes if supported by the specific database.