oracle查询:取出每组中的第一条记录

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

H***ING

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.