【Oracle】分析函数first、last、first_value、last_value

FIRST和LAST函数

功能：获取首行和尾行（可以有并列的情况）的非排序字段的值

语法：

aggregate_function KEEP (DENSE_RANK {FIRST |LAST }ORDERBY expr [DESC | ASC] [NULLSFIRST | LAST])

OVER (query_partition_clause)

使用说明：

①first和last函数有over子句就是分析函数，没有就是聚合函数。

②函数的参数必须是数字类型（或者其他类型可转为数字类型），返回相同类型

③aggregate_function可以是MIN,MAX,SUM,AVG,COUNT,VARIANCE,STDDEV

示例：

用表：SELECT *FROM hr.employees;

当作聚合函数用：按部门分组，按提成比例排序，取最大最小薪水

SELECT department_id

  ,MIN(salary)keep(dense_rankFIRST ORDER BY commission_pct) "Worst"

  ,MAX(salary)keep(dense_rankLAST ORDER BY commission_pct) "Best"

FROM hr.employees

GROUP BY department_id

ORDER BY department_id;

当作分析函数用：按部门分组，按提成比例排序，取最大最小薪水

SELECT last_name

 ,department_id

  ,salary

  ,MIN(salary)keep(dense_rankFIRSTORDER BY commission_pct)over(PARTITION BY department_id)"Worst"

  ,MAX(salary)keep(dense_rankLASTORDER BY commission_pct)over(PARTITION BY department_id)"Best"

FROM hr.employees

ORDER BY department_id,salary, last_name;

first_value()和last_value()函数

功能：FIRST_VALUE是一个返回一组排序值后的第一个值的分析函数。

语法：

【推荐】FIRST_VALUE( expr ) [RESPECT |IGNOREN ULLS] OVER ( analytic_clause )

【老版本】FIRST_VALUE( expr [RESPECT |IGNOREN ULLS])OVER ( analytic_clause )

使用说明：

①在计算时，默认考虑null，除非指定忽略null值。

示例：

60部门按照薪资排序，取第一行、当前行、最后一行

SELECT department_id

 ,employee_id

 ,last_name

  ,salary

 ,first_value(last_name) over(ORDER BY salary rows unbounded preceding)AS fv

 ,last_value(last_name) over(ORDER BY salary ROWS BETWEEN unbounded PRECEDING AND CURRENTROW ) AS lv

 ,last_value(last_name) over(ORDER BY salary ROWS BETWEEN unbounded PRECEDING AND UNBOUNDED FOLLOWING ) AS lv

FROM hr.employees

WHERE department_id =60;