Combine multiple rows into one space separated string

Question

So I have 5 rows like this

userid, col
--------------
1, a
1, b
2, c
2, d
3, e

How would I do query so it will look like this

userid, combined
1, a b
2, c d
3, e

Answer 1

Use the GROUP_CONCAT aggregate function:

  SELECT yt.userid,
         GROUP_CONCAT(yt.col SEPARATOR ' ') AS combined
    FROM YOUR_TABLE yt
GROUP BY yt.userid

The default separator is a comma (","), so you need to specify the SEPARATOR of a single space to get the output you desire.

If you want to ensure the order of the values in the GROUP_CONCAT, use:

  SELECT yt.userid,
         GROUP_CONCAT(yt.col ORDER BY yt.col SEPARATOR ' ') AS combined
    FROM YOUR_TABLE yt
GROUP BY yt.userid

Answer 2

In hive you can use

SELECT userid, collect_set(combined) FROM tabel GROUP BY user_id;

collect_set removes duplicated. If you need to keep them you can check this post:

COLLECT_SET() in Hive, keep duplicates?

Answer 3

SELECT 
  userid,
  concat_ws(" ", collect_set(col)) AS combined
FROM table 
GROUP BY userid

Answer 4

1. MySQL with duplicates: select col1, group_concat(col2) from table1 group by col1
2. MySQL without duplicates: select col1, group_concat(distinct col2) from table1 group by col1
3. Hive with duplicates: select col1, collect_list(col2) from table1 group by col1
4. Hive without duplicates: select col1, collect_set(col2) from table1 group by col1

Answer 5

I'm pretty sure that you can't do this using Hive QL. However, it should be possible to do so if you write your own Map/Reduce scripts - see this tutorial to get started.