SQL Server GROUP by has review

The difference between where and having is that where is the filter before grouping, and having is the filter after grouping

Example of a field restriction specified in Group by select 3

Select category, sum (quantity) as number of sums, summary from Agroup by category order BY category Desc

Example 3 will prompt for an error after execution, such as. This is a point to note that the field specified in select is either included in the GROUP BY statement, as a basis for grouping, or is included in an aggregate function .