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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
"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".
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.
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.
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.
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7, Group By and aggregate functionIn 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
sum(column name) | ||
---|---|---|
max(column name) | Maximum value | |
min(column name) | minimum value | |
avg(column name) | average Value | |
##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; 示例6:求各组记录数目 select 类别, count(*) AS 记录数 from A group by 类别; 示例7:求各组记录数目 8、Having与Where的区别
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 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 9、Compute 和 Compute Byselect * from A where 数量 > 8 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) 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 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:
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: 相关视频: |
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