how to use DISTINCT ON with mysql using ActiveRecord - mysql

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, time')
Getting SQL syntax error.
ActiveRecord::StatementInvalid (Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ON(user_id) user_id, time FROM `events` ORDER BY `events`.`time` DESC ' at line 1: SELECT DISTINCT ON(user_id) user_id, time FROM `events` ORDER BY `events`.`time` DESC LIMIT 11)
events table column names
["id", "visit_id", "user_id", "name", "properties", "time"]
can anyone help me out in this
Expected output something like this
{ 21 => "2018-12-18 09:44:59", 42 => "2018-12-19 12:08:59"}
21 is user_id and value is last visit time
Using mysql as database

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