PSEUDO Columns

A PSEUDO-column is an Oracle assigned value (pseudo-field) but not stored on disk.
PSEUDO columns are not actual columns in a table but they behave like columns.

For example, you can select values from a PSEUDO column. However, you cannot insert into, update, or delete from a PSEUDO column.

Automatically filled by oracle.
PSEUDO  columns are Hidden columns.

Below are the PSEUDO columns.

• SYSDATE
• NEXTVAL
• CURRVAL
• ROWID
• ROWNUM
• LEVEL
• SQLCODE
• SQLERRM
• NEW
• OLD

ROWID :

 It is automatically assigned with every row inserted  into table. It is an unique value.

 It is stored permanently in database.

 It comprises of Object id, Data file id, Block id &

 Record id.  It is Re usable.

 ROWNUM :

  It is an dynamic value automatically retrieved along with Select statement output.

  It is an unique value.

  It is only for display purpose.

  It is not stored in database.

LEVEL :

It will arrange the select statement output in Inverted tree structure ( Hierarichal Tree ) and gives the position of row in Tree. 

( Returns Number ) 

 It contains duplicate values.

 * Retrieving the Hierarichy of employees based on      their Superior.

Select Level,empno,ename,sal,job,mgr from emp

   connect by prior empno = mgr start with

   mgr is null order by Level;