how to use DISTINCT ON with mysql using ActiveRecord

Question

Want all the latest visit of all the distinct user.

For this I am using below query

Event.order(time: :desc).select(\'DISTINCT ON(user_id) user_id,          


        

Answer 1

So you want all user visits with last visited time.

Instead of use DISTINCT function, you can use GROUP with MAX function.

Query looks like

Events.group(:user_id).maximum(:time)

This Outputs your desired results

{21=>Tue, 18 Dec 2018 11:15:24 UTC +00:00, 23=>Thu, 20 Dec 2018 06:42:10 UTC +00:00}

Hope this works for you.

FYI DISTINCT ON(columns). is PostgreSQL Syntax.

Answer 2

Like i said in the comments DISTINCT ON(column), * is PostgreSQL SQL syntax.

The query below you need to have to rewrite in MySQL

SELECT 
 DISTINCT ON(user_id) AS user_id, time
FROM 
 <table>
ORDER BY 
 time DESC
LIMIT 11 

The most easy to do that is using SUBSTRING_INDEX(GROUP_CONCAT(..)).
The queries below should give you the correct results.

SET SESSION group_concat_max_len = @@max_allowed_packet;

SELECT 
   user_id
 , SUBSTRING_INDEX(
     GROUP_CONCAT(time
                  ORDER BY
                     time DESC
     )
     , ','
     , 1
 ) AS time
FROM 
 <table>
GROUP BY 
 user_id
ORDER BY 
 time DESC
LIMIT 11