MYSQL LEFT JOINing the latest record from another table - mysql

Question

I have a training_stats table (current due training) and I also have a completed_training table.
What I want to do is query due training with the last completed date from the completed table.
I've nearly got what I want, I get the due training, but they are duplicated with each completed record(as there are many completed records to each current due), and I only want single rows and the latest completed date.
I've been trying to use MAX, and when I run the MAX query independently, I get the last record. But when the MAX query is in the join, it is returning all completed rows.
This is the query that I am using:
SELECT s.course_stat_id
,o.org_name
,u.id
,u.first_name
,u.last_name
,a.area_id
,a.area_name
,tc.course_id
,tc.course_name
,s.assigned_on
,s.due
,s.pass_mark
,s.completed_on
,completed.complete_training_id
,completed.complete_date
FROM training_stats s
JOIN organisations o ON o.org_id = s.org_id
LEFT JOIN (
SELECT complete_training_id
,user_id
,area_id
,course_id
,max(completed_on) AS complete_date
FROM completed_training
GROUP BY complete_training_id
) completed ON completed.user_id = s.user_id
AND completed.area_id = s.area_id
AND completed.course_id = s.course_id
LEFT JOIN users u ON u.id = s.user_id
LEFT JOIN areas a ON a.area_id = s.area_id
LEFT JOIN training_courses tc ON tc.course_id = s.course_id
WHERE u.active = 1
AND o.active = 1
AND s.assigned = 1
Can you see what I am doing wrong?

Answer 1

Not exactly positive of your expected results, but the failure is PROBABLY for your group by and JOIN. Your group by is ONLY on the training ID, but you are also pulling user, area and course as well as max date completed for said respective training ID, user, area, course. You group by and join should match the unique characteristics.
Without seeing data, the query as I interpret it is that the "complete_training_id" is an auto-increment column for that table. Having said that, there would only ever be one record for that ID.
Having said that, the completed training table can have for a single user, area and course, multiple training days of which you want the most recent. For example someone attending college and needs to take many computer classes and they are refreshers from prior so assume all are same course ID. A person could take in 2012, 2014, 2016. You would want the instance of the user/area/course showing the 2016 dated training. So lets look at that first.
select
ct.user_id,
ct.area_id,
ct.course_id,
max(ct.completed_on) AS complete_date
FROM
completed_training ct
GROUP BY
ct.user_id,
ct.area_id,
ct.course_id
Now, for each user, area and course of study, I have one record with the most recent completion date. NOW lets pull the rest of the details, but since you need the completed training ID too, I applied the MAX() of that in the query below. The ID should by default be increasing every time a new record is added, so one completed a year ago would have a lower value than the ID completed today. So you get both the completed ID and its corresponding date for a given user, area, course.
SELECT
s.course_stat_id,
o.org_name,
u.id,
u.first_name,
u.last_name,
a.area_id,
a.area_name,
tc.course_id,
tc.course_name,
s.assigned_on,
s.due,
s.pass_mark,
s.completed_on,
ct.complete_training_id,
ct.complete_date
FROM
training_stats s
JOIN organisations o
ON s.org_id = o.org_id
AND o.active = 1
LEFT JOIN
( select
ct.user_id,
ct.area_id,
ct.course_id,
max(ct.complete_training_id ) as complete_training_id,
max(ct.completed_on) AS complete_date
FROM
completed_training ct
GROUP BY
ct.user_id,
ct.area_id,
ct.course_id ) ct
on s.user_id = ct.user_id
AND s.area_id = ct.area_id
AND s.course_id = ct.course_id
JOIN users u
ON s.user_id = u.id
AND u.active = 1
LEFT JOIN areas a
ON s.area_id = a.area_id
LEFT JOIN training_courses tc
ON s.course_id = tc.course_id
WHERE
s.assigned = 1

Answer 2

I'm not 100% sure of that. First, run this query. It should list all completed training, with a rnk from 1 (lastest), to n (oldest).
SELECT complete_training_id
,user_id
,area_id
,course_id
,completed_on AS complete_date
,#curRank := case when complete_training_id <> #cur_complete_training_id then 0 else #curRank + 1 end rnk
FROM completed_training, (select #curRank := 0, #cur_complete_training_id := 0)
ORDER BY complete_training_id, completed_on DESC
If true, the answer is :
SELECT s.course_stat_id
,o.org_name
,u.id
,u.first_name
,u.last_name
,a.area_id
,a.area_name
,tc.course_id
,tc.course_name
,s.assigned_on
,s.due
,s.pass_mark
,s.completed_on
,completed.complete_training_id
,completed.complete_date
FROM training_stats s
JOIN organisations o ON o.org_id = s.org_id
LEFT JOIN (
SELECT complete_training_id
,user_id
,area_id
,course_id
,completed_on AS complete_date
,#curRank := case when complete_training_id <> #cur_complete_training_id then 0 else #curRank + 1 end rnk
FROM completed_training, (select #curRank := 0, #cur_complete_training_id := 0)
ORDER BY complete_training_id, completed_on DESC
) completed ON completed.user_id = s.user_id and completed.rnk = 1
AND completed.area_id = s.area_id
AND completed.course_id = s.course_id
LEFT JOIN users u ON u.id = s.user_id
LEFT JOIN areas a ON a.area_id = s.area_id
LEFT JOIN training_courses tc ON tc.course_id = s.course_id
WHERE u.active = 1
AND o.active = 1
AND s.assigned = 1