SQL - Multiple join left with sum doesn't give expected result - sql

Question

Here is my request
SELECT j.* ,
c.name as client_name ,
s.name as supplier_name,
s.ID as supplier_id ,
mt.* ,
SUM(pb.require_followup) as nb_followup,
SUM(ws.worked_time) as hours_on_job,
SUM(iv.total) as total_price,
SUM(iv.hour_expected) as hours_planned,
j.ID as ID
FROM $wpdb->posts j
LEFT JOIN ".Job::$META_TABLE." mt ON mt.post_id = j.ID
LEFT JOIN ".Job::$LINK_TABLE_JOB_CONTACT." l1 ON l1.job_id = j.ID
LEFT JOIN ".Contact::$TABLE_NAME." c ON c.ID = l1.contact_id
LEFT JOIN ".Supplier::$TABLE_NAME." s ON s.ID = c.supplier_id
LEFT JOIN ".Problem::$TABLE_NAME." pb ON pb.job_id = j.ID
LEFT JOIN ".Worksheet::$TABLE_NAME." ws ON ws.job_id = j.ID
LEFT JOIN ".Invoice::$TABLE_NAME." iv ON iv.job_id = j.ID
WHERE j.post_status = 'publish'
AND j.post_type = 'job'
".implode(' ',$where_condition)."
GROUP BY j.ID
ORDER BY j.post_date DESC
the Problem is that result for SUM is wrong when I LEFT JOIN other table.
The row 53 for example give 105 for nb_followup instead of 1
Where this request return the right result simply by removing the last 2 LEFT JOIN : LEFT JOIN ".Worksheet::$TABLE_NAME." ws ON ws.job_id = j.ID and
LEFT JOIN ".Invoice::$TABLE_NAME." iv ON iv.job_id = j.ID
SELECT j.* ,
c.name as client_name ,
s.name as supplier_name,
s.ID as supplier_id ,
mt.* ,
SUM(pb.require_followup) as nb_followup,
j.ID as ID
FROM $wpdb->posts j
LEFT JOIN ".Job::$META_TABLE." mt ON mt.post_id = j.ID
LEFT JOIN ".Job::$LINK_TABLE_JOB_CONTACT." l1 ON l1.job_id = j.ID
LEFT JOIN ".Contact::$TABLE_NAME." c ON c.ID = l1.contact_id
LEFT JOIN ".Supplier::$TABLE_NAME." s ON s.ID = c.supplier_id
LEFT JOIN ".Problem::$TABLE_NAME." pb ON pb.job_id = j.ID
WHERE j.post_status = 'publish'
AND j.post_type = 'job'
".implode(' ',$where_condition)."
GROUP BY j.ID
ORDER BY j.post_date DESC
Also removing only LEFT JOIN ".Invoice::$TABLE_NAME." iv ON iv.job_id = j.ID will give 15 as result for the row 53
To resume
Full request give 105 -> wrong should be 1
removing the last join give 15 -> wrong should be 1
removing the last 2 join give 1 -> Correct

Answer 1

You need to calculate the SUM()s BEFORE you join, otherwise the rows multiply because of the joins and this in turn leads to errors in summation. e.g.
SELECT
j.ID as ID
, pb.nb_followup
FROM $wpdb->posts j
LEFT JOIN (select pb.job_id, SUM(pb.require_followup) as nb_followup from ".Problem::$TABLE_NAME." pb GROUP BY pb.job_id) pb ON pb.job_id = j.ID
The other problem you are facing is that MySQL permits "lazy syntax" for group by. Don't use this lazy syntax or you will get unexpected error/bugs. It is very simple to avoid, REPEAT every column of the select clause in the group by clause UNLESS the column is using an aggregate function such as SUM(), COUNT(), MIN(), MAX() and so on.e.g.
select a.col1, b.col2, c.col3 , sum(d.col4)
from a
inner join b on a.id = b.aid
inner join c on b.id = c.bid
inner join d on c.id = d.cid
group by a.col1, b.col2, c.col3