Composite Attributes in apex oracle
Not all logical attributes have to be present in the physical model, as you can easily produce them while querying data:

SELECT firstname || ' ' || lastname AS fullname
FROM person


If you insist on implementing it as a column, you could use a generated (computed) column:

CREATE TABLE PERSON
(
FIRSTNAME VARCHAR2(15) NOT NULL,
LASTNAME VARCHAR2(15)  NOT NULL,
FULLNAME VARCHAR2(31) GENERATED ALWAYS AS (FIRSTNAME || ' ' || LASTNAME) VIRTUAL, 
...


For completeness I should mention that you could also create a user-defined object type with FIRSTNAME and LASTNAME as its attributes, but I personally think that you shouldn't go that route for portability and maintainability reasons.