我必须在存储过程中创建多个表，

create or replace procedure test()
language sql 
as $$
  drop table if exists a1;
  create table a1 as
  select * from mastertable;
  
   drop table if exists a2;
  create table a2 as
  select * from a1 with some filter
  
   drop table if exists a3;
  create table a3 as
  select * from a2 with some filter
;$$

但它抛出了一个错误

ERROR:  relation mastertable" does not exist
LINE 40:       from mastertable

请帮我修好它。