What occurs when data is not entered into a relation as a result of an incomplete primary key?

When data is not entered into a relation due to an incomplete primary key, this scenario represents an insertion anomaly. An insertion anomaly occurs when the structure of a database prevents the addition of data to a database due to the lack of sufficient information, often tied to an incomplete primary key.

In the context of relational databases, a primary key is essential because it uniquely identifies each record in a table. If the primary key is incomplete, it might not be possible to insert new records without violating the constraints of uniqueness required by the primary key. For instance, if a primary key is made up of multiple attributes and one or more of those attributes are missing or null, the database will not allow a new entry that relies on that key. This situation leads to data loss because you cannot fully populate or insert new rows without all key components being present.

This concept underscores the importance of properly defining primary keys and ensuring that they are fully populated during data entry to prevent any unintended loss of data functionality.