创建表及序列时，先判断其是否存在
1、创建表时，先判断表是否存在，如果存在则删除
create or replace procedure pr_droptable(
    v_tablename        in varchar2
)
as
v_count int;
begin
    select count(1) into v_count from user_tables where table_name = upper(v_tablename);
    if v_count > 0 then
        execute immediate 'drop table ' || v_tablename;
    end if;
end;

2、创建序列时，先判断序列是否存在，如果存在则删除
create or replace procedure pr_dropseq(
    v_name        in varchar2
)
as
v_count int;
begin
    select count(1) into v_count from user_sequences a  where sequence_name = upper(v_name);
    if v_count > 0 then
        execute immediate 'drop sequence ' || v_name;
    end if;
end;