sequence可重复执行脚本

declare
v_rowcount integer;
begin
v_rowcount:=0;
select count(*) into v_rowcount from ALL_SEQUENCES
where sequence_name='SEQ_XX';
if v_rowcount=0 then
 execute immediate 'create sequence SEQ_XX 
   minvalue 1
   maxvalue 9999999999
   start with 1
   increment by 1
   cache 20
   order';
end if;
end;