之前整理过一篇文章《从oracle去重复记录实例中学习使用group by， having 》，可以和今天这篇放在一起去理解oracle查询时的的分组和排序。

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。这里还有一点要注意的是：select 后面列的选择，一定要选适合分组的列，不然会达不到分组的效果。

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.