Question

I have 3 tables: problems, categories, and a joining table for problem_categories

I am trying to get and count all the categories associated with a problem, and count how many problems are associated with each problem.

Here is my query which doesn’t return correct results:

SELECT 
  category_name , 
  categories.category_id , 
  problems.problem_id, 
  COUNT(problems.problem_id) as num_problems 
FROM 
  problem_categories 
left JOIN 
  categories 
on 
  problem_categories.category_id = categories.category_id 
left join 
  problems 
on
  problem_categories.problem_id = problems.problem_id 
WHERE 
  problem_categories.problem_id = 266
GROUP BY 
  problems.problem_id , category_name

but as soon as I take out the where clause, it returns all the categories and correct counts of problems. But the trick is that I need to query this for a particular problem_id

Any way to adjust this query to get it right? What am I doing incorrectly here?

Thanks!!

Answer 1

Your question is somewhat unclear. How can you count problems if you are selecting a specific one? If you want to count categories associated with a particular problem you do this (I am assuming you have foreign keys and no duplicates in the problem_categories table):

SELECT problem_id, COUNT(category_id) as num_categories
FROM problem_categories
WHERE problem_id = 266
GROUP BY problem_id

then join to problem table if you need to print out more problem information (for efficiency, always get the detail last). If you are wanting to count problem -> category -> problem then do:

select p1.problem_id, (COUNT(DISTINCT(p2.problem_id)) - 1) as num_problems
FROM
(select problem_id, category_id FROM problem_categories where problem_id = 266) as p1
inner join 
(select problem_id, category_id FROM problem_categories) as p2
ON p1.category_id = p2.category_id
GROUP by p1.problem_id

Again if you want to show the problem details, join to the problem table to get them. If you want the num_problems and num_categories together join the first query to the second one before joining to the problem table to get the problem detail. i.e.:

SELECT p1.problem_id, p1.num_categories, p2.num_problems 
FROM
(SELECT problem_id, COUNT(category_id) as num_categories
FROM problem_categories
WHERE problem_id = 266
GROUP BY problem_id) as p1
INNER JOIN
(SELECT p1.problem_id, (COUNT(DISTINCT(p2.problem_id)) - 1) as num_problems
FROM
(SELECT problem_id, category_id FROM problem_categories where problem_id = 266) as p1
INNER JOIN 
(SELECT problem_id, category_id FROM problem_categories) as p2
ON p1.category_id = p2.category_id
GROUP by p1.problem_id) as p2
ON p1.problem_id = p2.problem_id