oracle分析函数：三、first,first_value,lag,lead函数

/************************************************
**分析函数-First/Last函数
**时间：2016-7-8
*************************************************/
--1 语法
aggregate_function keep
( dense_rank first order by expr desc nulls last)
over (query partition clause )
--说明
•DENSE_RANK FIRST or DENSE_RANK LAST indicates that Oracle Database will 
aggregate over only those rows with the minimum (FIRST) or the maximum (LAST) dense rank (also called olympic rank).
--**简介：


FIRST and LAST are very similar functions. 
Both are aggregate and analytic functions that operate on a set of values from a set of rows that rank as the FIRST or LAST 
with respect to a given sorting specification. 
都聚集和分析功能，给定一个排序的规范，操作一组值从一组行排名第一或最后一个；
If only one row ranks as FIRST or LAST, then the aggregate operates on the set with only one element.


--aggregate functions：
忽略over clause
--analytic functions 
指定over clause
-------------------------------------------------
--2 分析例子
SELECT unit_id,
       unit_code,
       tjrq_q,
       MIN(out_row) KEEP(DENSE_RANK FIRST ORDER BY tjrq_q) OVER(PARTITION BY unit_id) "Worst",
       MAX(out_row) KEEP(DENSE_RANK LAST ORDER BY tjrq_q) OVER(PARTITION BY unit_id) "Best"
  FROM etl_exdw_log
 ORDER BY unit_id, unit_code;


--3 DENSE_RANK FIRST order by expr和dens_rank() over
SELECT unit_id,
       unit_code,
       tjrq_q,
       MIN(out_row) KEEP(DENSE_RANK FIRST ORDER BY tjrq_q) OVER(PARTITION BY unit_id) "Worst",
       MAX(out_row) KEEP(DENSE_RANK LAST ORDER BY tjrq_q) OVER(PARTITION BY unit_id) "Best",
       dense_rank() over(partition by unit_id order by out_row),
       rank() over(partition by unit_id order by out_row)
  FROM etl_exdw_log
 ORDER BY unit_id, unit_code;
-------------------------------------------------
--4 聚集函数例子
SELECT unit_id,
       unit_code,
       tjrq_q,
       MIN(out_row) KEEP(DENSE_RANK FIRST ORDER BY tjrq_q) "Worst",
       MAX(out_row) KEEP(DENSE_RANK LAST ORDER BY tjrq_q) "Best",
       dense_rank(23632) within group(order by out_row desc nulls last)
  FROM etl_exdw_log
 group BY unit_id, unit_code, tjrq_q;


/************************************************
**分析函数-First_value/Last_value函数
**时间：2016-7-8
**简介：
*************************************************/
--1 语法
--ANSI format(推荐):
firt_value (expr (respect|ignore) nulls ) over (analytic clause)
或者：
firt_value (expr) (respect|ignore) nulls  over (analytic clause)
-------------------------------------------------
--2 简介
It returns the first value in an ordered set of values;
If the first value in the set is null, then the function returns NULL unless you specify IGNORE NULLS
-------------------------------------------------
--3 例子一
SELECT unit_id,
       unit_code,
       tjrq_q,
       out_row,
       FIRST_VALUE(out_row) OVER(partition by unit_id,tjrq_q ORDER BY out_row desc ROWS UNBOUNDED PRECEDING) AS greatest_rows
  FROM etl_exdw_log
 ORDER BY unit_id,tjrq_q;
/************************************************
**分析函数-lag/lead函数
**时间：2016-7-8
**简介：
*************************************************/
--1 语法1
lag (value_expr,offset,default) (ignore nulls| respect nulls)
over (query_partition_clause order_by_clause)
--2 语法2
lag (value_expr (ignore nulls| respect nulls),offset,default) 
over (query_partition_clause order_by_clause)
-------------------------------------------------
--3 目的
LAG provides access to a row at a given physical offset prior to that position.
已知偏移量，根据给的物理便宜量，lag函数可以获得另外一个列
Given a series of rows returned from a query and a position of the cursor
在一个查询结果集中，返回一个游标的位置和一组行


可选的偏移量参数,指定一个整数大于零。如果你不指定偏移量,那么其默认是1。如果偏移量超出窗口范围可选的默认值(1)返回;


{RESPECT | IGNORE} NULLS determines whether null values of value_expr are included in or eliminated from the calculation. The default is RESPECT NULLS.


You cannot nest analytic functions by using LAG or any other analytic function for value_expr. 
不能嵌套分析


--lag (value_expr,offset,default) (ignore nulls| respect nulls) over (query_partition_clause order_by_clause)
-------------------------------------------------
--4 例子
SELECT unit_id,
       unit_code,
       start_time,
       end_time,
       LAG(end_time, 1, sysdate) OVER(partition by unit_id ORDER BY end_time) AS prev_endtime,
       round((end_time - LAG(end_time, 1, sysdate)
              OVER(partition by unit_id ORDER BY end_time)) * 24 * 60) as exe_time
  FROM ETL_EXDW_LOG
 order by unit_id, end_time;
------------------------------------------------
--5 lead函数对比：
SELECT unit_id,
       unit_code,
       start_time,
       end_time,
       lead(end_time, 1, sysdate) OVER(partition by unit_id ORDER BY end_time) AS prev_endtime,
       round((lead(end_time, 1, sysdate) OVER(partition by unit_id ORDER BY end_time)-end_time) * 24 * 60) as exe_time
  FROM ETL_EXDW_LOG
 order by unit_id, end_time;