SQL query wrong result - mysql

Question

i have this query:
SELECT `completed`.`ID` AS `ID`,`completed`.`level` AS `level`,`completed`.`completed_in` AS `completed_in`, COUNT(1) AS `right_answers_num`
FROM `completed`
INNER JOIN `history` ON `history`.`ID` = `completed`.`ID`
INNER JOIN `questions` ON `questions`.`ID` = `history`.`question`
WHERE `completed`.`student_id` = '1' AND `questions`.`answer` = `history`.`answer`
GROUP BY `completed`.`ID`
ORDER BY `completed`.`completed_in` DESC
what i need is to get info of each test in completed table (id,level,completed_in,right_answer_num)
the problem with that query is that if there is no one right answer(history.answer = questions.answer) then it doesn't return the row, while it should return the row(id,level,completed_in) and the right_answer_num(counter) should be zero..
please help me,, thanks ahead.

Answer 1

SELECT
completed.ID AS ID,
completed.level AS level,
completed.completed_in AS completed_in,
COUNT(questions.answer) AS right_answers_num
FROM completed
INNER JOIN history ON history.ID = completed.ID
LEFT JOIN questions ON questions.ID = history.question AND questions.answer = history.answer
WHERE
completed.student_id = '1'
GROUP BY
completed.ID
ORDER BY completed.completed_in DESC

Answer 2

use a LEFT OUTER JOIN intead of an INNER JOIN.

Answer 3

The second inner join is what's causing rows with no record in the questions table to be omitted. An inner join will only return rows that have data in all corresponding tables. Change the second inner join to a left join like so:
SELECT
completed.ID AS ID,
completed.level AS level,
completed.completed_in AS completed_in,
COUNT(questions.answer) AS right_answers_num
FROM completed
INNER JOIN history ON history.ID = completed.ID
LEFT JOIN questions ON questions.ID = history.question
WHERE completed.student_id = 1
GROUP BY completed.ID
ORDER BY completed.completed_in DESC