Multiple SQL joins difficult query - mysql

Question

I have a weird schema but when it was designed it seemed like a good idea at the time. I have one master table, lesson_objects, that has foreign keys linking to the vocabulary, video and quizzes tables.
vocab table:
CREATE TABLE `se_vocab` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`vocab_word` VARCHAR(255) NOT NULL,
`vocab_audio` INT(10) NULL DEFAULT '0',
`vocab_image` INT(10) NULL DEFAULT '0',
PRIMARY KEY (`id`)
)
video table:
CREATE TABLE `se_video` (
`id` INT(10) NOT NULL AUTO_INCREMENT,
`video_name` VARCHAR(255) NOT NULL,
`video_description` MEDIUMTEXT NOT NULL,
`video_file_name` VARCHAR(50) NULL DEFAULT NULL,
`video_url` VARCHAR(255) NOT NULL,
PRIMARY KEY (`id`)
)
quizzes table:
CREATE TABLE `se_quizzes` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`quiz_name` VARCHAR(80) NOT NULL,
`quiz_description` TINYTEXT NULL,
PRIMARY KEY (`id`)
)
lesson objects (contains foreign keys of previous tables)
CREATE TABLE `se_lesson_org` (
`id` INT(10) NOT NULL AUTO_INCREMENT,
`lesson_id` INT(10) NOT NULL,
`section_object_type` ENUM('video','vocabulary','quiz') NOT NULL,
`section_object_id` INT(10) NOT NULL,
PRIMARY KEY (`id`)
)
I'm trying to create a query that returns all the records from lesson_objects but also includes the data in the columns for the type in that record (vocabulary, etc.)
For example:
Only my query returns no rows, while ideally it should turn multiple rows with every record containing SOME empty columns. E.g. if it isn't vocabulary, the columns for quiz and video will be empty.
My attempts are very bad, but here is one for the sake of guidance:
SELECT
lo.id, lo.section_object_type, lo.section_object_id,
vo.id, vo.vocab_text, vo.vocab_image, vo.vocab_audio
vi.id, vi.video_name, vi.video_url,
q.id, q.quiz_name
FROM se_lesson_org lo, se_vocab vo, se_video vi, se_quizzes q
WHERE lo.section_object_id = vo.id
OR lo.section_object_id = vi.id
OR lo.section_object_id = q.id
Any help / comments would be appreciated. Thanks.

Answer 1

Use LEFT JOIN to return all rows from se_lesson_org. Additionally, add a JOIN condition to match for the specific section_object_type
SELECT
lo.*,
vo.*,
vi.*,
q.*
FROM se_lesson_org lo
LEFT JOIN se_vocab vo
ON vo.id = lo.section_object_id
AND lo.section_object_type = 'vocabulary'
LEFT JOIN se_video vi
ON vi.id = lo.section_object_id
AND lo.section_object_type = 'video'
LEFT JOIN se_quizzes q
ON q.id = lo.section_object_id
AND lo.section_object_type = 'quiz'
Note: Avoid using old-style JOIN syntax. Read this article by Aaron Bertrand.

Answer 2

Sounds like you are looking for a LEFT JOIN on each of these tables. That way if the foreign key is valid you will show values and if it isn't valid you will just get NULL for the related columns.
SELECT
lo.id, lo.section_object_type, lo.section_object_id,
vo.id, vo.vocab_text, vo.vocab_image, vo.vocab_audio
vi.id, vi.video_name, vi.video_url,
q.id, q.quiz_name
FROM se_lesson_org lo
LEFT JOIN se_vocab vo ON vo.id = lo.section_object_id
AND lo.section_object_type = 'vocabulary'
LEFT JOIN se_video vi ON vi.id = lo.section_object_id
AND lo.section_object_type = 'video'
LEFT JOIN se_quizzes q ON q.id = lo.section_object_id
AND lo.section_object_type = 'quiz'
Notice also how this syntax makes it clear how each table is being connected into the rest of the query rather than having a whole mess of conditions in the WHERE clause at the end.