Question

Edit: Deleted my old message cause it was confusing. And i can’t answer my own question for now.

Found, problem come from GROUP BY.

After some researches, i found that we can’t use GROUP BY for group a column inside grouped rows.

So this work as expected :

SELECT candidats.*
      , AVG(test_results.rate_good_answer) AS toto
  FROM "candidats" 
LEFT JOIN "sessiontests"
  ON "sessiontests"."candidat_id" = "candidats"."id"  
LEFT JOIN "test_results"
  ON "test_results"."sessiontest_id" = "sessiontests"."id" 
LEFT JOIN "questionnaires"
  ON "questionnaires"."id" = "test_results"."questionnaire_id"  
WHERE (sessiontests.terminate = 't' )
  AND ("questionnaires"."category" LIKE '%java%' ) 
GROUP BY candidats.id 
ORDER BY toto

But this will grouped only my column in test_results :

SELECT candidats.*, 
  FROM "candidats" 
LEFT JOIN "sessiontests"
  ON "sessiontests"."candidat_id" = "candidats"."id"  
LEFT JOIN "test_results"
  ON "test_results"."sessiontest_id" = "sessiontests"."id" 
LEFT JOIN "questionnaires"
  ON "questionnaires"."id" = "test_results"."questionnaire_id"  
WHERE (sessiontests.terminate = 't' )
 AND ("questionnaires"."category" LIKE '%java%' ) 
GROUP BY candidats.id, test_results.rate_good_answer 
ORDER BY AVG(test_results.rate_good_answer)

Edit 3 :

My problem was the second query was returning each different test_results row for my candidats, whereas i expected it to return me one line per candidat.

First query, is the answer, and it works nice.

Answer 1

SELECT candidats.*
, AVG(test_results.rate_good_answer) AS toto
FROM “candidats”
LEFT JOIN “sessiontests”
ON “sessiontests”.”candidat_id” = “candidats”.”id”
LEFT JOIN “test_results”
ON “test_results”.”sessiontest_id” = “sessiontests”.”id”
LEFT JOIN “questionnaires”
ON “questionnaires”.”id” = “test_results”.”questionnaire_id”
WHERE (sessiontests.terminate = ‘t’ )
AND (“questionnaires”.”category” LIKE ‘%java%’ )
GROUP BY candidats.id
ORDER BY toto