group by with order by query issue

Question

I have activities table and sample data like

I need to fetch distinct contact_id order by created_at desc of activity_type_id 3

I have tried

select distinct contact_id 
from activities 
where activity_type_id = 3 order by created_at desc limit 40 offset 0

then postgres gives error like

ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list LINE 1: ...om activities where activity_type_id = 3 order by created_at...

I solved this kind of errors and tried queries but result in not as i expect.

Group by contact_id with order by created_at desc is not giving the correct result.

like above I tried, group by, order by, distinct etc combination of queries.

Please suggest me.

I want O/P like below from above data

   id  | contact_id | activity_type_id | created_at
-------+------------+------------------+---------------------
718006 |   45816    |         3        | 2016-10-03 12:29:57
718005 |   45153    |         3        | 2016-10-03 12:27:01
718204 |    3491    |         3        | 2016-10-03 12:25:14
718186 |   42393    |         3        | 2016-10-03 12:23:00
718172 |   26867    |         3        | 2016-10-03 12:21:07
718171 |   45954    |         3        | 2016-10-03 12:20:09

basically all unique contact_id order by created_at desc order.

Thanks

Answer 1

You need to specify which contact id you care about -- because you have multiple rows.

When you use select distinct , only the columns in the select are available in the order by clause.

Hence, use group by and order by like this:

select a.contact_id
from activities a
where a.activity_type_id = 3
group by a.contact_id
order by max(a.created_at) desc
limit 40 offset 0;

Answer 2

What about this ?

SELECT contact_id, MAX(created_at) AS CreatedAt 
FROM activities 
WHERE activity_type_id = 3 
GROUP BY contact_id 
ORDER BY CreatedAt DESC
limit 40 offset 0