【转】oracle查询:取出每个分组中的第一条记录

排序后取第一条记录
select * from (select * from table order by a )C
where rownum=1

oracle查询:取出每组中的第一条记录
按type字段分组,code排序,取出每组中的第一条记录

 

方法一：

 

select type,min(code) from group_info 
group by type;

 

注意：select 后面的列要在group by 子句中，或是用聚合函数包含，否则会有语法错误。

 

方法二：

 

SELECT * FROM(
SELECT z.type , z.code ,ROW_NUMBER()
OVER(PARTITION BY z.type ORDER BY z.code) AS code_id
FROM group_info z
)
WHERE code_id =1;

 

这里涉及到的over()是oracle的分析函数

 

参考sql reference文档：

 

Analytic functions compute an aggregate value based on a group of rows. They differ from aggregate functions in that they return multiple rows for each group.

 

Analytic functions are the last set of operations performed in a query except for the final ORDER BY clause. All joins and all WHERE , GROUP BY , and HAVING clauses are completed before the analytic functions are processed. Therefore, analytic functions can appear only in the select list or ORDER BY clause.

 

语法结构：

 

analytic_function ([ arguments ]) OVER
 (analytic_clause)

其中analytic_clause结构包括：

 

[ query_partition_clause ]
[ order_by_clause [ windowing_clause ] ]

 

也就是：函数名（ [ 参数 ] ） over（ [ 分区子句 ]  [ 排序子句 [ 滑动窗口子句 ] ]）

 

     这里PARTITION BY 引导的分区子句类似于聚组函数中的group by，排序子句可看成是select语句中的order by.

===================================================================================

对多数据进行分组排序后取每组第一条记录

 

SELECT *
  FROM (SELECT ROW_NUMBER() OVER(PARTITION BY <PK_FINANCE_ACCOUNT_1> ORDER BY <amassdate> DESC) LEV,
               <hx_amass_liquid>.*
          FROM <hx_amass_liquid>)
 WHERE LEV = 1
 ORDER BY <PK_FINANCE_ACCOUNT_2> DESC

 

<PK_FINANCE_ACCOUNT_1> -- 分组列 

 

<amassdate> -- 排序列

 

<hx_amass_liquid> -- 表名

 

<PK_FINANCE_ACCOUNT_2> -- 排序列

 

注：<hx_amass_liquid>.* -- 一定要有表名，否则会报“缺失表达式”错误

 

==========================================================

 

分组时不仅想取每组的第一条记录，还想取每组共有几条记录

 

SELECT *
FROM (

 

SELECT ROW_NUMBER() OVER(PARTITION BY PK_FINANCE_ACCOUNT ORDER BY amassdate DESC) LEV,
            

             count(0) over(partition by PK_FINANCE_ACCOUNT) cnt,

             

             hx_amass_liquid.*
 FROM

 

 hx_amass_liquid

 

)

 WHERE LEV = 1
 ORDER BY PK_FINANCE_ACCOUNT DESC