MySQL 查询 - 如何根据多列内连接的一列获取唯一行

Question

I have a query which selects members and the groups they are in for an organization. So a member can be in multiple groups. The purpose of the query is to get all members that are in the selected groups. But I only want a member returned once, along with only one of the group names. I do not want all the group names they belong to.

For instance below is list of members and the groups they are in:
Mbr Grp
1 -- G1
1 -- G2
2 -- G2
2 -- G3
3 -- G4

The desired result is a member and one group they are in:
1 -- G1
2 -- G2
3 -- G4

I have the following query working which returns the list of members and groups they are in, for each member / group combination:

```
SELECT ogm.group_id, ogm.member_id,  
      FROM org_group_member ogm 
INNER JOIN org_group og ON ogm.group_id
     WHERE ogm.group_id IN ('1', '2', '3')
  GROUP BY ogm.group_id, ogm.member_id
  ORDER BY ogm.group_id

```

This results in the first list I showed above. But I want the desired list.
I know how to limit query to unique member ids, but I do not know how to include only group id.

What must I add to my query to achieve the results I desire?

Answer 1

You can use aggregation:

SELECT ogm.member_id, ANY_VALUE(ogm.group_id),   
FROM org_group_member ogm JOIN
     org_group og
     ON ogm.group_id
WHERE ogm.group_id IN (1, 2, 3)
GROUP BY ogm.member_id;

Note that the group_id is probably a number, which is why I removed the single quotes. Only use single quotes for string and date constants.

Answer 2

I found the answer to my question. I should have only included the ogm.member_id in the GROUP BY clause. Then I only got each member once with the first group they were found in.

SELECT ogm.group_id, ogm.member_id,  
      FROM org_group_member ogm 
INNER JOIN org_group og ON ogm.group_id
     WHERE ogm.group_id IN ('1', '2', '3')
  GROUP BY ogm.member_id
  ORDER BY ogm.group_id