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.