GROUP BY 子句没有聚合函数有什么原因吗？
我现在正在（彻底）学习 SQL，并遇到了 GROUP BY 子句。
GROUP BY 根据您提供的参数对结果集进行聚合或分组。如果您在查询中使用此子句，则可以对结果集执行聚合函数以查找结果集的统计信息，例如查找平均值 (AVG()) 或频率 (COUNT())< /代码>。
我的问题是：如果没有附带的聚合函数，GROUP BY 语句是否有用？
更新 使用 GROUP BY 作为 DISTINCT 的同义词（可能）是一个坏主意，因为我怀疑它会更慢。
I'm (thoroughly) learning SQL at the moment and came across the GROUP BYclause.
GROUP BY aggregates or groups the resultset according to the argument(s) you give it. If you use this clause in a query you can then perform aggregate functions on the resultset to find statistical information on the resultset like finding averages (AVG()) or frequency (COUNT()).
My question is: is the GROUP BY statement in any way useful without an accompanying aggregate function?
Update
Using GROUP BY as a synonym for DISTINCT is (probably) a bad idea because I suspect it is slower.
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在这种情况下，使用
DISTINCT将是同义词，但您想要/必须定义GROUP BY子句的原因是为了能够定义 < code>HAVING 子句详细信息。如果您需要定义
HAVING子句，则必须定义GROUP BY- 您不能这样做它与DISTINCT结合使用。Using
DISTINCTwould be a synonym in such a situation, but the reason you'd want/have to define aGROUP BYclause would be in order to be able to defineHAVINGclause details.If you need to define a
HAVINGclause, you have to define aGROUP BY- you can't do it in conjunction withDISTINCT.您可以使用不带任何AGGREGATES的GROUP BY来执行DISTINCT选择。
You can perform a DISTINCT select by using a GROUP BY without any AGGREGATES.
分组依据主要可以两种方式使用
1) 与 SQL 聚合函数结合
2）从结果集中消除重复行
所以您的问题的答案位于上述用途的第二部分。
Group by can used in Two way Majorly
1)in conjunction with SQL aggregation functions
2)to eliminate duplicate rows from a result set
SO answer to your question lies in second part of USEs above described.
注意：以下所有内容仅适用于
MySQLGROUP BY是 保证按顺序返回结果，DISTINCT则不然。GROUP BY与ORDER BY NULL的效率与DISTINCT相同（并以 say 方式实现）。如果正在聚合（或区分）的字段上有索引，则两个子句都对该字段使用松散索引扫描。在
GROUP BY中，您可以返回非分组和非聚合表达式。MySQL将从相应组中选取任意随机值来计算表达式。使用
GROUP BY，您可以省略SELECT子句中的GROUP BY表达式。使用DISTINCT，你就做不到。DISTINCT返回的每一行都保证是唯一的。Note: everything below only applies to
MySQLGROUP BYis guaranteed to return results in order,DISTINCTis not.GROUP BYalong withORDER BY NULLis of same efficiency asDISTINCT(and implemented in the say way). If there is an index on the field being aggregated (or distinctified), both clauses use loose index scan over this field.In
GROUP BY, you can return non-grouped and non-aggregated expressions.MySQLwill pick any random values from from the corresponding group to calculate the expression.With
GROUP BY, you can omit theGROUP BYexpressions from theSELECTclause. WithDISTINCT, you can't. Every row returned by aDISTINCTis guaranteed to be unique.它不仅仅用于聚合函数。
例如，考虑以下代码：
这将仅返回每个产品 1 个结果，但包含该记录的最新更新值。
It is used for more then just aggregating functions.
For example, consider the following code:
This will return only 1 result per product, but with the latest updated value of that records.