Pseudocolumns are not actual columns in a table, but they behave like columns. They can be thought of as special-purpose data elements that can be used in SQL statements just as if they were part of the table, but are not actually contained in a table. Essentially, a pseudo-column is an Oracle assigned value used in the same context as an Oracle Database column, but not stored on disk.
You can select values from a pseudocolumn but you cannot INSERT INTO, UPDATE, or DELETE from a pseudo-column. Pseudocolumns are allowed in SQL statements, but not in procedural statements. SQL and PL/SQL recognize the following SQL pseudocolumns, which return specific data items (this is a partial list):