The DUAL table is a special one-row, one-column table present by default in all Oracle database installations. Originally, the database engine would perform disk I/O on the DUAL table, but beginning with Oracle 10g Release 1, no physical or logical I/O is performed on DUAL. DUAL is readily available for all users in the database. The CREATE SEQUENCE syntax is demonstrated, creating a sequence named SEQ1 that starts at 1000, increments by 1, does not cache, has a maximum value of 1005, and cycles when the maximum is reached. The NEXTVAL pseudocolumn is used to retrieve the next value from the sequence.

1. Part 1- create sequence
DUAL TABLE
1DR. GIRIJA NARASIMHAN

2. The DUAL table is a special one-row, one-column table present by
default in all Oracle database installations.
DUAL is originally a table and the database engine would
perform disk IO on the table when selecting from DUAL. This disk
IO was usually logical IO (not involving physical disk access) as
the disk blocks were usually already cached in memory. This
resulted in a large amount of logical IO against the DUAL table.
Beginning with 10g Release 1 Oracle Database have been
optimized and the database no longer performs physical or
logical IO on the DUAL table even though the DUAL table still
actually exists.
DUAL is readily available for all the users in database. It is not
limited to system administrator only.
2DR. GIRIJA NARASIMHAN

3. DR. GIRIJA NARASIMHAN 3
create sequence Syntax

4. SQL> create sequence seq1
2 START WITH 1000
3 INCREMENT BY 1
4 NOCACHE
5 maxvalue 1005
6 cycle;
Sequence created.
4

5. SQL> select seq1.nextval from dual;
NEXTVAL
----------
1004
SQL> select seq1.nextval from dual;
NEXTVAL
----------
1005
SQL> select seq1.nextval from dual;
NEXTVAL
----------
1
SQL> select seq1.nextval from dual;
NEXTVAL
----------
2
SQL> select seq1.nextval from dual;
NEXTVAL
----------
1000
SQL> select seq1.nextval from dual;
NEXTVAL
----------
1001
SQL> select seq1.nextval from dual;
NEXTVAL
----------
1002
SQL> select seq1.nextval from dual;
NEXTVAL
----------
1003 5DR. GIRIJA NARASIMHAN