备注:测试数据库版本为MySQL 8.0

如需要scott用户下建表及录入数据语句，可参考:
scott建表及录入数据sql脚本

一.需求

原表数据:
±-------±-------+
| deptno | ename |
±-------±-------+
| 20 | SMITH |
| 30 | ALLEN |
| 30 | WARD |
| 20 | JONES |
| 30 | MARTIN |
| 30 | BLAKE |
| 10 | CLARK |
| 20 | SCOTT |
| 10 | KING |
| 30 | TURNER |
| 20 | ADAMS |
| 30 | JAMES |
| 20 | FORD |
| 10 | MILLER |
±-------±-------+

要求输出为:
| 10 | CLARK,KING,MILLER |
| 20 | SMITH,JONES,SCOTT,ADAMS,FORD |
| 30 | ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES |

二.解决方案

使用MySQL自带的group_concat函数即可

代码:

select deptno,
       group_concat(ename order by empno separator  ',' ) as emps
  from emp
group by deptno

测试记录

mysql> select deptno,
    ->        group_concat(ename order by empno separator  ',' ) as emps
    ->   from emp
    -> group by deptno;
+--------+--------------------------------------+
| deptno | emps                                 |
+--------+--------------------------------------+
|     10 | CLARK,KING,MILLER                    |
|     20 | SMITH,JONES,SCOTT,ADAMS,FORD         |
|     30 | ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES |
+--------+--------------------------------------+
3 rows in set (0.00 sec)