What does group by mean in SQL?

Group by in SQL is to group data according to the rules specified by "By", and the so-called grouping is to divide a "data set" into several "small areas", and then target several "small areas" area" for data processing.

group_by means which field or fields to group the data according to by. "Group By" literally means to group data according to the rules specified by "By". The so-called grouping is to divide a "data set" into several "small areas", and then target several "small areas". data processing.

Grammar structure is as follows:

select   字段    from   表名   where    条件     group   by       字段
或者
select   字段    from   表名   group  by    字段    having    过滤条件

Note: For filtering conditions, you can use where first, then group by or group by first, then having

Original table

Simple Group By

Example 1

select category, sum(quantity) as sum of quantities

from A

group by category

The return result is as shown in the table below, which is actually a summary of categories.

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