GROUP BY如何运作? [英] How does GROUP BY work?

问题描述

假设我有一个表Tab1，该表具有属性-a1，a2，...等.所有属性都不是唯一的.

Suppose I have a table Tab1 with attributes - a1, a2, ... etc. None of the attributes are unique.

以下查询的性质是什么?它会始终返回单行吗?

What will be the nature of the following query? Will it return a single row always?

SELECT a1, a2, sum(a3) FROM Tab1 GROUP BY a1, a2

推荐答案

GROUP BY为GROUP BY字段的每个唯一组合返回一行.因此，在您的示例中，出现在Tab1行中的每个(a1, a2)不同组合都会导致查询中的一行代表具有给定group by字段值组合的行组.像SUM()这样的聚合函数是在每个组的成员上计算的.

GROUP BY returns a single row for each unique combination of the GROUP BY fields. So in your example, every distinct combination of (a1, a2) occurring in rows of Tab1 results in a row in the query representing the group of rows with the given combination of group by field values . Aggregate functions like SUM() are computed over the members of each group.

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