DB7.2以后才有sequence这个对象，创建的时候和oralce一样。
CREATE SEQUENCE seq_name AS INT
START WITH 1
INCREMENT BY 1
MINVALUE 1
NO MAXVALUE
NO CYCLE
NO CACHE
ORDER
在使用的过程中有些不同
DB2中：nextval for sequencename ，prevval for sequencename
select nextval for seq_name from sysibm.sysdummy1;
select prevval for seq_name from sysibm.sysdummy1;
oracle中：sequencename.nextval，sequencename.currval
select seq_name.nextval from dual;
select seq_name.currval from dual;
sysibm.sysdummy1是DB2内存中的一个特殊的表。
