Can one part of a composite primary key have a null value?

In a composite primary key in a database, none of the parts can have a null value. This is because the primary key must uniquely identify a record, and null indicates the absence of a value, which violates data uniqueness and integrity.

In Java, if you use a composite key, for example in JPA, all fields that are part of the composite key must not be null.

Example of a composite key in JPA:

@Embeddable
public class OrderItemId implements Serializable {
    private Long orderId;
    private Long productId;
    // getters, setters, equals, and hashCode
}

@Entity
public class OrderItem {
    @EmbeddedId
    private OrderItemId id;
    // other fields
}

The values of orderId and productId must be filled and cannot be null.