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The system organizes the usage of Group By in SQL and the restrictions on multiple fields of Group By

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Release： 2018-08-03 17:06:57
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When to use Group By in SQL? This article explains in detail the usage of Group By. Its simple definition is to divide a "data set" into several "small areas", and then perform data processing on several "small areas". What are the restrictions on the fields specified by Select in Group By? apache php mysql

1. Overview

"Group By" literally means to group data according to the rules specified by "By". The so-called grouping is to group a "data set" Divide it into several "small areas", and then perform data processing on several "small areas".

2. Original table

3. Simple Group By

Example 1

select 类别, sum(数量) as 数量之和
from A
group by 类别
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The return result is as follows, In fact, it is a summary.

4, Group By and Order By

Example 2

select 类别, sum(数量) AS 数量之和
from A
group by 类别
order by sum(数量) desc
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The return results are as follows

"order by sum of quantities desc" cannot be used in Access, but it can be used in SQL Server.

5. Field restrictions specified by Select in Group By

Example 3

select 类别, sum(数量) as 数量之和, 摘要
from A
group by 类别
order by 类别 desc
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After executing Example 3, an error will be prompted, as shown below. This is what needs to be noted. The fields specified in select must either be included after the Group By statement as the basis for grouping; or they must be included in the aggregate function.

6、Group By All

Example 4

select 类别, 摘要, sum(数量) as 数量之和
from A
group by all 类别, 摘要
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In Example 4, you can specify the "Summary" field because " "Multi-column grouping" contains "summary fields", and the execution results are as follows Group the values. In Example 4, you can see that "a, a2001, 13" is the merger of the two records "a, a2001, 11" and "a, a2001, 2".

Although "group by all" is supported in SQL Server, GROUP BY ALL will be deleted in future versions of Microsoft SQL Server to avoid using GROUP BY ALL in new development work. Access does not support "Group By All", but Access also supports multi-column grouping. The above SQL in SQL Server can be written in Access as 

select 类别, 摘要, sum(数量) AS 数量之和
from A
group by 类别, 摘要
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7, Group By and aggregate function

In Example 3, it is mentioned that the field specified by select in the group by statement must be the "grouping field". If other fields want to appear in the select, they must be included in the aggregate function. Common aggregate functions are as follows:

Function

FunctionSupportsum(column name) Sum　　max(column name)Maximum value　　min(column name)minimum value　　avg(column name)average Value　　Only Access supportsOnly supported by AccessNote the difference from count(*)
##first(column name)First record
last(column name)Last record
count(column name)Statistical record number

示例5：求各组平均值

select 类别, avg(数量) AS 平均值 from A group by 类别;
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示例6：求各组记录数目

select 类别, count(*) AS 记录数 from A group by 类别;
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示例7：求各组记录数目

8、Having与Where的区别

  • where 子句的作用是在对查询结果进行分组前，将不符合where条件的行去掉，即在分组之前过滤数据，where条件中不能包含聚组函数，使用where条件过滤出特定的行。

  • having 子句的作用是筛选满足条件的组，即在分组之后过滤数据，条件中经常包含聚组函数，使用having 条件过滤出特定的组，也可以使用多个分组标准进行分组。

示例8

select 类别, sum(数量) as 数量之和 from A
group by 类别
having sum(数量) > 18
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示例9：Having和Where的联合使用方法

select 类别, SUM(数量)from A
where 数量 gt;8
group by 类别
having SUM(数量) gt; 10
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9、Compute 和 Compute By

select * from A where 数量 > 8
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执行结果：

示例10：Compute

select *
from A
where 数量>8
compute max(数量),min(数量),avg(数量)
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执行结果如下：

compute子句能够观察“查询结果”的数据细节或统计各列数据（如例10中max、min和avg），返回结果由select列表和compute统计结果组成。

示例11：Compute By

select *
from A
where 数量>8
order by 类别
compute max(数量),min(数量),avg(数量) by 类别
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执行结果如下：

示例11与示例10相比多了“order by 类别”和“... by 类别”，示例10的执行结果实际是按照分组（a、b、c）进行了显示，每组都是由改组数据列表和改组数统计结果组成，另外：

  • compute子句必须与order by子句用一起使用

  • compute...by与group by相比，group by 只能得到各组数据的统计结果，而不能看到各组数据

在实际开发中compute与compute by的作用并不是很大，SQL Server支持compute和compute by，而Access并不支持

相关文章：

sql group by 语句用法

sql group by语法与实例

相关视频：

SQL趣味课堂

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