pseudocolumn in Oracle

A pseudocolumn or Pseudo-column is like a table column but it actually not exist or sotred in the table, so you can select from pseudocolumns to read its value but you can't insert, update or delete their values.


Examples:

The table employees has only 3 columns as we can see by using description command
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SQL> desc employees;
 Name                            Null?    Type
 ------------------------------- -------- ----
 ID                                       NUMBER
 NAME                                     VARCHAR2(20)
 SALARY                                   NUMBER(7,2)

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SQL> select ID,NAME,SALARY from employees;
 
       ID NAME                    SALARY
--------- -------------------- ---------
      190 Thomas                    3500
      253 Abraham                   2700
      262 Charlie                   2700
      267 Louis                     2850
      182 John                      3000
      520 William                   2700
 
6 rows selected.

ROWNUM:

returns a number indicating the order in which a row was selected from a table.
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SQL> select ROWNUM,ID,NAME,SALARY from employees;
 
   ROWNUM        ID NAME                    SALARY
--------- --------- -------------------- ---------
        1       190 Thomas                    3500
        2       253 Abraham                   2700
        3       262 Charlie                   2700
        4       267 Louis                     2850
        5       182 John                      3000
        6       520 William                   2700
 
6 rows selected.



ROWNID:

returns the address in the database of each row was selected from a table.
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SQL> select ROWID,ID,NAME,SALARY from employees;
 
ROWID                     ID NAME                    SALARY
------------------ --------- -------------------- ---------
AAADYsAAEAAAAD1AAA       190 Thomas                    3500
AAADYsAAEAAAAD1AAB       253 Abraham                   2700
AAADYsAAEAAAAD1AAC       262 Charlie                   2700
AAADYsAAEAAAAD1AAD       267 Louis                     2850
AAADYsAAEAAAAD1AAE       182 John                      3000
AAADYsAAEAAAAD1AAF       520 William                   2700
 
6 rows selected.


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