用法：group_concat (待拼接对象)
输出：用逗号进行拼接后的字符串
select group_concat(emp_no) as employees from dept_emp;
/*
结果：
employees |
+-------------------------------------------------------------------+
| 10001,10002,10003,10004,10005,10006,10007,10008,10009,10010,10010 |
+-------------------------------------------------------------------+
*/
标签：group,+-------------------------------------------------------------------+,拼接,M
From： https://www.cnblogs.com/jk200/p/17121588.html