select语句后加上分号试试？

这样就可以，这声明一个变量来判断if和用exists有区别吗

可以这样试试

declare
    begin
  if exists (select * from aa.aa where a1='cc') then
      select * from aa.aa where a1='cc' ;
    else
    select count(*) from aa.aa where a1='cc' ;
    end if;
    end