Usage of some functions in sql statement

一、group by：https://blog.csdn.net/hengji666/article/details/54924387

2. Use of group_conca and SUBSTRING_INDEX : https://blog.csdn.net/u011280342/article/details/78285608

3. The use of group by with having and order by must follow certain principles:

                       1. The columns of order by and having must be columns that appear in the group by clause

                     2.order by, having should be placed behind group by