oracle中lead和lag函式

這兩個函式，是偏移量函式，其用途是：可以查出同一欄位下一個值或上一個值。

lead(col_name,num,flag)
col_name是列名；num是取向下第幾個值；flag是一個標誌，也就是如果向下第幾個值是空值的話就取flag；
例如lead(login_time,1,null)這個是向下取一個值，如果這個值為空則按空算，當然也可以用其他值替換。
lag(col_name,num,flag)
和lead類似，col_name是列名；num是取向上第幾個值；flag是一個標誌，也就是如果向上第幾個值是空值的話就取flag；
例如lag(login_time,1,null)這個是向上取一個值，如果這個值為空則按空算，當然也可以用其他值替換。

舉個例子：有一個表tmp_test(u_id,login_time)查一下這個表中連續7天都有登入機器的人是誰？
造下資料：
create table tmp_test(u_id number,login_time date);

insert into tmp_test
select 1 rn,sysdate + rownum as login_time
from dual
connect by level <=8
union
select 2 rn,sysdate + rownum as login_time
from dual
connect by level <=3
union
select 3 rn,sysdate + rownum as login_time
from dual
connect by level <=2
union
select 2 rn,sysdate + rownum+4 as login_time
from dual
connect by level <=5;
commit;

然後造幾條重複資料：
insert into tmp_test
select 1 rn,sysdate + rownum as login_time
from dual
connect by level <=3;

查下資料：
select * from tmp_test;

 U_ID LOGIN_TIME
---------- -----------
 1 2012/3/8 6:33:24
 1 2012/3/9 6:33:24
 1 2012/3/10 6:33:24
 1 2012/3/11 6:33:24
 1 2012/3/12 6:33:24
 1 2012/3/13 6:33:24
 1 2012/3/14 6:33:24
 1 2012/3/15 6:33:24
 2 2012/3/8 6:33:24
 2 2012/3/9 6:33:24
 2 2012/3/10 6:33:24
 2 2012/3/12 6:33:24
 2 2012/3/13 6:33:24
 2 2012/3/14 6:33:24
 2 2012/3/15 6:33:24
 2 2012/3/16 6:33:24
 3 2012/3/8 6:33:24
 3 2012/3/9 6:33:24
 1 2012/3/8 6:37:35
 1 2012/3/9 6:37:35
 1 2012/3/10 6:37:35

從上面資料看出其實只有u_id=1滿足條件，那麼怎麼用sql實現呢？
SQL> select distinct u_id
 2 from (select u_id,
 3 login_time last_login_time,
 4 lead(login_time, 6) over(partition by u_id order by u_id, login_time) next_login_time
 5 from (select distinct u_id, trunc(login_time) login_time
 6 from tmp_test))
 7 where next_login_time - last_login_time = 6;

 U_ID
----------
 1

ok，就是這個結果。其實用lag也可以實現相同結果，寫法如下：

select distinct u_id
 from (select u_id,
 login_time last_login_time,
 lag(login_time, 6) over(partition by u_id order by u_id, login_time) next_login_time
 from (select distinct u_id, trunc(login_time) login_time
 from tmp_test))
 where last_login_time - next_login_time = 6;