检索用户名：select user from dual
删除当前用户下所有表的内容：
declare
-- 指向所有 table 的游标
cursor c_t is
select table_name
from user_tables;
table_name user_tables.table_name%type;
begin
open c_t;
loop
fetch c_t into table_name;
exit when c_t%notfound;
-- 用 delete 而不用 truncate 是为了能户用户回滚,减少误操作
execute immediate 'delete from ' || table_name;
end loop;
close c_t;
end;