oracle first_value，last_valus

first_value和last_value 是用来去分析函数窗口中对应列的第一个值和最后一个值的函数。

语法如下：

first_value(col [ignore NULLS]) over([PARTITION BY col] [ORDER BY sal] [windows])
last_value(col [ignore NULLS]) over([PARTITION BY col] [ORDER BY sal] [windows])
--col          : 表示选取的列
--ignore NULLS :表示忽略空值
--PARTITION BY :表示分组
--ORDER BY     :表示排序
--windows      :表示窗口，默认值是（RANGE UNBOUNDED PRECEDING AND CURRENT ROW）从第一行到当前行
--windows      :常用值（ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING）从第一行到最后一行               

示例1：

SELECT first_value(comm ) over() firstval,
       last_value(comm ) over() lastval,
       e.*
  FROM emp e

结果：

示例2：

SELECT first_value(comm ) over(partition by deptno) firstval,
       last_value(comm ) over(partition by deptno) lastval,
       e.*
  FROM emp e

结果：

示例3：

SELECT first_value(comm ) over(partition by deptno order by sal) firstval,
       last_value(comm ) over(partition by deptno order by sal) lastval,
       e.*
  FROM emp e

结果：

示例4：

SELECT first_value(comm ignore nulls) over(partition by deptno order by sal) firstval,
       last_value(comm ignore nulls) over(partition by deptno order by sal) lastval,
       e.*
  FROM emp e

结果：

示例5：

SELECT first_value(sal ignore nulls) over(partition by deptno order by sal) firstval,
       last_value(sal ignore nulls) over(partition by deptno order by sal) lastval,
       e.*
  FROM emp e;

SELECT first_value(sal ignore nulls) over(partition by deptno order by sal) firstval,
       last_value(sal ignore nulls) over(partition by deptno order by sal ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING ) lastval,
       e.*
  FROM emp e

结果：

第一个sql结果：

第二个sql结果：