Question

this is my SQL question

Table – Activity

id activity userID

0 99 1

1 99 2

2 99 3

3 88 1

4 77 2

5 77 3

I hope to query this table to generate such a result which pools the choice of activity by

different users:

99 1 2 3

88 1

77 2 3

My first attempt is using JOIN to self-join the table recursively. But it is a headache. Anyone has other suggestion?

Thanks

Edit:
Another problem. How about if I want to check whether users are friends before grouping?

Table – Friends

id userID friendID
0 1 2
1 2 1
2 1 3
3 3 1
4 2 3
5 3 2

So they are friends (in both directions). Then I want to do the pooling (GROUP BY) given that they are friends again.

Again, thanks. You guys offer me great guidance.

Answer 1

Use GROUP BY activity and the GROUP_CONCAT()

Like this code:

SELECT activity,GROUP_CONCAT(userID,' ') AS Groups
FROM Activity
GROUP BY activity
ORDER BY activity DESC;

SEE THE FIDDLE