数据表：user
| id | name | group_id |
| 1 | 张三 | 1 |
| 2 | 李四 | 1 |
| 3 | 王五 | 2 |
1. mysql、sqlite：group_contact函数:
select group_concat(id) ids, group_id from user group by group_id
结果：
| ids | group_id |
| 1,2 | 1 |
| 3 | 2 |
2.mssql：stuff函数 + FOR XML PATH
select stuff((select ','+convert(varchar(25), id) from user as t2 where t2.group_id = t1.group_id FOR XML PATH('')), 1, 1, '' ) as ids, group_id from user as t1 group by group_id
结果：
| ids | group_id |
| 1,2 | 1 |
| 3 | 2 |