日期:2014-05-16 浏览次数:20446 次
按type字段分组,code排序,取出每组中的第一条记录
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方法一:
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select type,min(code) from group_info
group by type;
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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。
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方法二:
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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;
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这里涉及到的over()是oracle的分析函数
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参考sql reference文档:
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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.
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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.
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语法结构:
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analytic_function ([ arguments ]) OVER (analytic_clause)
其中analytic_clause结构包括:
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[ query_partition_clause ] [ order_by_clause [ windowing_clause ] ]
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也就是:函数名( [ 参数 ] ) over( [ 分区子句 ] [ 排序子句 [ 滑动窗口子句 ] ])
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???? 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.