Question

I have this query (that works fine in MySQL):

SELECT units.*, activity_logs.created_at 
FROM "units" left join activity_logs on units.id=activity_logs.unit_id 
GROUP BY units.id 
ORDER BY activity_logs.created_at desc

Postgres gives this error:

PG::Error: ERROR:  column "activity_logs.created_at" must appear in the GROUP BY clause or be used in an aggregate function

I know that Postgres is right SQL-wise but how do I get what I want? That is the latest activity_log entry for each unit joined with all of the units table?

Thanks in advance!

• Jacob

Answer 1

The error explains it: you need to use an aggregate function to make this deterministic, eg:

SELECT units.*, max(activity_logs.created_at)
FROM "units" left join activity_logs on units.id=activity_logs.unit_id 
GROUP BY units.id 
ORDER BY max(activity_logs.created_at) desc;