Question

I have this working query which works correctly:

select problems.problem_id , problem_title , sum( vote ) as totalVotes 
from problems 
left join problem_votes on 
problems.problem_id = problem_votes.problem_id 
left join problem_categories on 
problems.problem_id = problem_categories.problem_id  
where problem_categories.category_id = 1 GROUP BY problem_title;  

when I add another join on a suggested_solutions table and a sum(column_in_suggested_solutions_table) to see how many suggested solutions there are for a problem, the query looks like this:

select problems.problem_id , problem_title , sum( vote ) as totalVotes , count(solution_name) from problems 
left join problem_votes on 
problems.problem_id = problem_votes.problem_id 
left join problem_categories on 
problems.problem_id = problem_categories.problem_id  
left join suggested_solutions on
problems.problem_id = suggested_solutions.problem_id
where problem_categories.category_id = 1 GROUP BY problem_title;

The problem with the second query is that while it still returns the same number of rows, the original sum() function returns an inflated number of counts, and the count function for suggested_solutions also returns an inflated number.

Any idea what I am doing wrong here?

Thanks!

Answer 1

There are multiple suggested_solutions rows for a particular problem_id which means that the rows getting summed in your original query get inflated.

You can do the group by in a derived table and join onto that instead.

SELECT problems.problem_id,
       problem_title,
       SUM(vote) AS totalVotes,
       Solution_Count
FROM   problems
       LEFT JOIN problem_votes
         ON problems.problem_id = problem_votes.problem_id
       LEFT JOIN problem_categories
         ON problems.problem_id = problem_categories.problem_id
       LEFT JOIN (SELECT COUNT(solution_name) AS Solution_Count,
                         problem_id
                  FROM   suggested_solutions
                  GROUP  BY problem_id) ss
         ON problems.problem_id = ss.problem_id
WHERE  problem_categories.category_id = 1
GROUP  BY problem_title;  

Note that also your WHERE problem_categories.category_id = 1 converts the first LEFT JOIN between problem_votes and problems to an INNER JOIN