1.建表

-- Create table
create table FRUIT
(
  id      VARCHAR2(20),
  name    VARCHAR2(20),
  class   VARCHAR2(20),
  count   VARCHAR2(20),
  buydate VARCHAR2(20)
)
tablespace USERS
  pctfree 10
  initrans 1
  maxtrans 255
  storage
  (
    initial 64K
    minextents 1
    maxextents unlimited
  );

insert into fruit (ID, NAME, CLASS, COUNT, BUYDATE)
values ('1', '蘋果', '水果', '10', '2011-7-1');

insert into fruit (ID, NAME, CLASS, COUNT, BUYDATE)
values ('1', '桔子', '水果', '20', '2011-7-2');

insert into fruit (ID, NAME, CLASS, COUNT, BUYDATE)
values ('1', '香蕉', '水果', '15', '2011-7-3');

insert into fruit (ID, NAME, CLASS, COUNT, BUYDATE)
values ('2', '白菜', '蔬菜', '12', '2011-7-1');

insert into fruit (ID, NAME, CLASS, COUNT, BUYDATE)
values ('2', '青菜', '蔬菜', '19', '2011-7-2');
 

--1	1	香蕉	水果	15	2011-7-3
--2	2	青菜	蔬菜	19	2011-7-2

select * from fruit where (id,buydate) in (select id,max(buydate) from fruit group by id);

本例也可以使用exists來實現：

select * from FRUIT t where not exists(select * from FRUIT where id=t.id and buydate>t.buydate);
 

在這裡只想說，in和exists的原理相似，二者應該是能夠實現相同的查詢效果的，只是寫法不同罷了。另外，本例是否能夠通過rownum和其他高階函式實現呢，留待以後研究吧。