一个简单的group_concat就可以完成这个任务：

select it.TeammateID,
       group_concat( distinct it.Teammatename SEPARATOR ',') as Teammatename,
       group_concat( it.Teamid SEPARATOR ',') as Teamid,
       group_concat( it.Teams SEPARATOR ',') as Teams
from intercom_teammates it
group by it.TeammateID ;

https://dbfiddle.uk/rU8-h8rX

• 注意 *。我在Teammatename上使用了distinct，但我认为这是多余的，如果每个不同的TeammateID的Teammatename都是唯一的，则可以删除它。

SELECT
    tm.teammate_id AS "Teammate ID",
    tm.name AS "Teammate name",
    GROUP_CONCAT(DISTINCT itt.team_id ORDER BY itt.team_id ASC) AS "Team IDs",
    GROUP_CONCAT(DISTINCT it.team ORDER BY it.team DESC) AS "Teams"
FROM intercom_teammates tm
LEFT JOIN intercom_teammate_teams itt ON tm.teammate_id = itt.teammate_id
LEFT JOIN intercom_teams it ON tm.teammate_id = it.teammate_id
GROUP BY tm.teammate_id, tm.name

注意：我已经将您的JOIN更正为intercom_teams，将itt.teammate_id更改为it.teammate_id;它会丢弃结果集。
结果：

| Teammate ID | Teammate name | Team IDs | Teams                    |
|-------------|---------------|----------|--------------------------|
| 1           | Amy           | 11,12,13 | Support,Sales,Marketing  |
| 2           | Peter         | 12,13    | Support,Marketing        |

• 小提琴 *