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

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

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

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

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

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

Function

	##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; Copy after login 示例6：求各组记录数目 select 类别, count(*) AS 记录数 from A group by 类别; Copy after login 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 Copy after login 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 Copy after login 9、Compute 和 Compute By select * from A where 数量 > 8 Copy after login 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) Copy after login 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 Copy after login 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 The above is the detailed content of The system organizes the usage of Group By in SQL and the restrictions on multiple fields of Group By. For more information, please follow other related articles on the PHP Chinese website! Statement of this Website The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn Hot Article How Long Does It Take To Beat Split Fiction? 3 weeks ago By DDD Repo: How To Revive Teammates 3 weeks ago By 尊渡假赌尊渡假赌尊渡假赌 Hello Kitty Island Adventure: How To Get Giant Seeds 3 weeks ago By 尊渡假赌尊渡假赌尊渡假赌 R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal) 1 weeks ago By 尊渡假赌尊渡假赌尊渡假赌 Difficulty in updating caching of official account web pages: How to avoid the old cache affecting the user experience after version update? 3 weeks ago By 王林 Show More Hot tools Tags Code&IT Voice Business Marketing AI Detector Chatbot Design&Art Hot Article How Long Does It Take To Beat Split Fiction? 3 weeks ago By DDD Repo: How To Revive Teammates 3 weeks ago By 尊渡假赌尊渡假赌尊渡假赌 Hello Kitty Island Adventure: How To Get Giant Seeds 3 weeks ago By 尊渡假赌尊渡假赌尊渡假赌 R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal) 1 weeks ago By 尊渡假赌尊渡假赌尊渡假赌 Difficulty in updating caching of official account web pages: How to avoid the old cache affecting the user experience after version update? 3 weeks ago By 王林 Show More Hot Article Tags Notepad++7.3.1 Easy-to-use and free code editor SublimeText3 Chinese version Chinese version, very easy to use Zend Studio 13.0.1 Powerful PHP integrated development environment Dreamweaver CS6 Visual web development tools SublimeText3 Mac version God-level code editing software (SublimeText3) Show More Hot Topics Where is the login entrance for gmail email? 7137 9 Java Tutorial 1534 14 Laravel Tutorial 1257 25 PHP Tutorial 1205 29 CakePHP Tutorial 1155 46 Show More Related knowledge Reduce the use of MySQL memory in Docker Mar 04, 2025 pm 03:52 PM Reduce the use of MySQL memory in Docker How do you alter a table in MySQL using the ALTER TABLE statement? Mar 19, 2025 pm 03:51 PM How do you alter a table in MySQL using the ALTER TABLE statement? How to solve the problem of mysql cannot open shared library Mar 04, 2025 pm 04:01 PM How to solve the problem of mysql cannot open shared library What is SQLite? Comprehensive overview Mar 04, 2025 pm 03:55 PM What is SQLite? Comprehensive overview Run MySQl in Linux (with/without podman container with phpmyadmin) Mar 04, 2025 pm 03:54 PM Run MySQl in Linux (with/without podman container with phpmyadmin) Running multiple MySQL versions on MacOS: A step-by-step guide Mar 04, 2025 pm 03:49 PM Running multiple MySQL versions on MacOS: A step-by-step guide How do I secure MySQL against common vulnerabilities (SQL injection, brute-force attacks)? Mar 18, 2025 pm 12:00 PM How do I secure MySQL against common vulnerabilities (SQL injection, brute-force attacks)? How do I configure SSL/TLS encryption for MySQL connections? Mar 18, 2025 pm 12:01 PM How do I configure SSL/TLS encryption for MySQL connections? See all articles