11. PSEUDO COLUMNS

PSEUDO COLUMNS:

    They are the false columns or dummy columns which behaves as same to that of table columns. They are actually functions.

•          SYSDATE
•      ROWNUM
•         ROWID
•         USER
•      UID
•      LEVEL
•      NEXTVAL
•         CURRVAL
•      CONNECT_BY_ISLEAF
•      CONNECT_BY_ISCYCLE
•          XML PSEUDO COLUMNS
•      COLUMN_VALUE
•      OBJECT_VALUE

        SYSDATE: Displays the system date or server date

            EG:  SELECT SYSDATEL, ENAME FROM EMP;

                   SELECT SYSDATE FROM EMP;

                   SELECT SYSDATE FROM DUAL;

                
                 ROWNUM:

•          ROWNUM provides the sequential number to the rows.
•      They are temporary numbers only to that query

 ROWID:

•          It is the physical address of the row
•          It is  in  hexadecimal ,generated for each of the row useful to identify the records and to increase the performance and they are permanent
•           They differ even for duplicate records also

EG:  SELECT ROWID, EMP.* FROM EMP;

       DELETE FROM EMP WHERE ROWID=’ROWIDVALUE’;

       SELECT * FROM EMP WHERE ROWID=’ROWIDVALUE;

Note: Indexes will make use of ROWID to increase the performance

   USER AND UID: They specify username and USERID

                 SELECT USER, UID FROM DUAL;