fetch需要显式声明游标，显式打开、关闭游标。 for不需要显式声明游标，是隐式打开、关闭游标。 1、fetch循环
open cursor_a; loop fetch cursor_a into rec; exit when cursor_a%not found; ...... end loop; close cursor_a
2、for循环 for rec in cursor_a loop ...... end loop;
3、单纯循环n次 for i in 1...n loop ..... end loop;
4、批量处理 open cursor_a; fetch cursor_a bulk collect into rec_list; ...... CLOSE cursor_a;
