我们通过这个文章来看一下分析函数LAST_VALUE的功效。
【Analytic】分析函数之MIN函数：http://space.itpub.net/519536/viewspace-624736
【Analytic】分析函数之MAX函数：http://space.itpub.net/519536/viewspace-624749
【Analytic】分析函数之AVG函数：http://space.itpub.net/519536/viewspace-624799
【Analytic】分析函数之ROW_NUMBER函数：http://space.itpub.net/519536/viewspace-624886
【Analytic】分析函数之RANK函数：http://space.itpub.net/519536/viewspace-624985
【Analytic】分析函数之DENSE_RANK函数：http://space.itpub.net/519536/viewspace-625115
【Analytic】分析函数之COUNT函数：http://space.itpub.net/519536/viewspace-625191
【Analytic】分析函数之FIRST_VALUE函数：http://space.itpub.net/519536/viewspace-625280
1.万变不离其宗，先看LAST_VALUE分析函数的10g语法描述。
http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/functions073.htm#i83648
LAST_VALUE(expr [ IGNORE NULLS ])
OVER (analytic_clause)
2.通过实验看一下分析函数LAST_VALUE的使用方法
1）创建测试表T，并初始化9条数据。
sec@ora10g> create table t (group_id number(10), name varchar2(10), salary int);
sec@ora10g> insert into t values (1,'Tom',1200);
sec@ora10g> insert into t values (2,'Kary',2400);
sec@ora10g> insert into t values (2,'Joe',800);
sec@ora10g> insert into t values (3,'Andy',100);
sec@ora10g> insert into t values (3,'Ellen',200);
sec@ora10g> insert into t values (3,'Erick',300);
sec@ora10g> insert into t values (3,'Hou',400);
sec@ora10g> insert into t values (3,'Mary',500);
sec@ora10g> insert into t values (3,'Secooler',600);
sec@ora10g> commit;
2）T表全貌
sec@ora10g> select * from t order by group_id,name;
GROUP_ID NAME SALARY
---------- ------------------------------ ----------
1 Tom 1200
2 Joe 800
2 Kary 2400
3 Andy 100
3 Ellen 200
3 Erick 300
3 Hou 400
3 Mary 500
3 Secooler 600
9 rows selected.
共三组数据，group_id分别是1、2和3。第1组有一个人，第2组有两个人，第3组有六个人。后一列是每个人的薪水值。
3）LAST_VALUE分析函数的简单用法
（1）在T表中添加一列，标识每一个数据分区中薪水高的人名。
sec@ora10g> col highest_sal_name for a16
sec@ora10g> select group_id, name, salary, LAST_VALUE(name) OVER (partition by group_id order by salary ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) as highest_sal_name from t order by group_id, name;
GROUP_ID NAME SALARY HIGHEST_SAL_NAME
---------- ------------------------------ ---------- ----------------
1 Tom 1200 Tom
2 Joe 800 Kary
2 Kary 2400 Kary
3 Andy 100 Secooler
3 Ellen 200 Secooler
3 Erick 300 Secooler
3 Hou 400 Secooler
3 Mary 500 Secooler
3 Secooler 600 Secooler
9 rows selected.
注意其中“ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING”的使用，若省略效果如下。
sec@ora10g> select group_id, name, salary, LAST_VALUE(name) OVER (partition by group_id order by salary) as highest_sal_name from t order by group_id,name;
GROUP_ID NAME SALARY HIGHEST_SAL_NAME
---------- ------------------------------ ---------- ----------------
1 Tom 1200 Tom
2 Joe 800 Joe
2 Kary 2400 Kary
3 Andy 100 Andy
3 Ellen 200 Ellen
3 Erick 300 Erick
3 Hou 400 Hou
3 Mary 500 Mary
3 Secooler 600 Secooler
9 rows selected.
显然这不是我们想要的效果:(，这是为什么呢~~~？给您一次思考和回答的机会。
如果对UNBOUNDED PRECEDING和UNBOUNDED FOLLOWING不熟悉，请参考Oracle官方文档“windowing_clause”http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/functions001.htm#i97640。
（2）在T表中添加一列，标识每一个数据分区中薪水高的薪水值。
sec@ora10g> col highest_sal_name for 9999
sec@ora10g> select group_id, name, salary, LAST_VALUE(SALARY) OVER (partition by group_id order by salary ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) as highest_sal_name from t order by group_id, name;
GROUP_ID NAME SALARY HIGHEST_SAL_NAME
---------- ------------------------------ ---------- ----------------
1 Tom 1200 1200
2 Joe 800 2400
2 Kary 2400 2400
3 Andy 100 600
3 Ellen 200 600
3 Erick 300 600
3 Hou 400 600
3 Mary 500 600
3 Secooler 600 600
9 rows selected.
3.小结
分析函数LAST_VALUE可以非常便利并迅速的的得到排序后的后一条数据的各种信息。但，细节之处不容忽视，细心使用。
有关分析函数的扩展可以参考Oracle的官方文档中的“Analytic Functions”描述：http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/functions001.htm#SQLRF06174
Good luck.
secooler
10.01.16
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【Analytic】分析函数之LAST_VALUE函数
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