Tricky Triple Sql Join While finding the most recent date

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I apologize if this question has been asked in a similar way before. I am rather new to SQL and am having a very difficult time finding the solution to my problem. Very confused.

I currently have three tables:


Templates - | id | name |


Users - | id | full_name |


Changelog - | id | id_user | id_template | last_edited_at


I have a table filled with templates.

I have a separate table full of users WHO make changes on those templates. Every time a change is made, an entry is created in the changelog table.

I am trying to create a query with the main goal being fetching EVERY record from templates, as well as WHEN it was most recently changed, and BY WHOM.

I came up with this query which allows me to find the most recent change for a specific template ID.

SELECT changelogs.id_user, changelogs.updated_at, users.full_name
FROM changelogs
JOIN users
ON changelogs.id_user = users.id
WHERE changelogs.id_template = :templateId
ORDER BY changelogs.updated_at DESC 
LIMIT 1

I orignally tried to solve this problem by looping through each template record, and running the above query for each record ID, but that is slow and doesn't work. I know there must be a way to do this in SQL, and I wanted to see if anybody has solved a similar problem. I didn't design this database, so sorry if it is not optimized.

My ideal final result would be a table that looks like:


| template.id | template.name | user.name | changelog.updated_at |

Thanks so much in advance for any guidance

You can use two JOINs to get the information from all the tables and then use a SUBQUERY to get the maximum value for the updated_at field, like this:

SELECT 
    t.id as template_id, t.name as template_name, 
    u.name as user_name, 
    c.updated_at as changelog_updated_at 
FROM 
    changelogs c

INNER JOIN templates t on c.id_template = t.id
INNER JOIN users u on c.id_user = u.id

WHERE 
  c.updated_at = 
    (SELECT MAX(updated_at) FROM changelogs c2 where c2.id_template = t.id)

ORDER BY t.template_name 

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You can use a correlated subquery:

SELECT cl.id_user, cl.updated_at, u.full_name
FROM changelogs cl JOIN
     users u
     ON cl.id_user = u.id
WHERE cl.updated_at = (SELECT MAX(cl2.updated_at)
                       FROM changelogs cl2
                       WHERE cl2.id_template = cl.templateId
                      );

For each template referred to in the outer query the subquery finds the maximum update date. This is then used to filter the records in the outer query.

How to join three tables in SQL query – MySQL , Joining multiple tables in SQL is always a tricky task, It can be more difficult if you need SQL Joins have always been tricky not only for new programmers but for Here is a general SQL query syntax to join three or more table. Let us know if you face any issue while running this 3 table JOIN query in any other database. A common query that crops up in web apps is finding the oldest or the most recent record in a single table. This is straightforward in SQL. You can even write the relevant part of the query without knowing anything about the table (other than the fact that it has a timestamp column called created_at):

I think a window function is your friend here. I didn't spin up sample tables to run this on, so there my be a type-o or two:

select *
from (

    select *
        ,row_number() over (partition by t.id order by c.last_edited_at desc) row
        from Templates t
        left join ChangeLog c
        on t.id=c.id_template
        left join Users u
        on c.id_user=u.id

) t
where row=1

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Comments
  • Wasn't me, only thing I noticed with this solution is that a template that was never changed isn't included. If the creation isn't included in the ChangeLog table.
  • @EdCallahan . . . Doesn't seem likely because the accepted answer also assumes that all templates are in the changelog.
  • I just noticed I was down-voted too. Whatever, how do you down-vote the use of a window function? They are simply cool. ;)
  • @EdCallahan is there anyway to run that window function (with row_number) or similar in MySQL, I can't get your query to run and I would like to set up a query that also includes templates that haven't been changed for another part of my project