Inner Join returning no results - mysql

Question

I need to get rows out from article and users table when the slug appears in my highlight table.
Highlight Table
id | slug
1 blue
2 green
Article Table
id | slug | title
1 blue
2 pink
User Table
id | slug | name
1 blue
2 green
3 brown
Heres my query:
SELECT slug from highlight_table
INNER JOIN article_table ON highlight_table.slug = article_table.slug
INNER JOIN user_table ON highlight_table.slug = user_table.slug
I would hope to get id 1 from article table and id 1 and 2 from users table.
The issue is Im getting nothing back from the query.

Answer 1

The query has an error because your SELECT slug is ambiguous. Your column slug appears in all of your tables so MySQL doesn't know which column to return. You need to do
SELECT `highlight_table`.`slug` from `highlight_table`
This will tell MySQL to only return the slug column from the highlight_table.
You should then only get 1 row which is blue, because blue exists in all three tables. Changing to LEFT JOIN for both article and user tables would get you 2 results back (green and blue) as INNER JOIN basically works as an AND and LEFT JOIN works more like an OR
Update!
Based on the final lot of information here is a query that does work:
SELECT highlight.slug from highlight
LEFT JOIN article ON highlight.slug = article.slug
LEFT JOIN user ON highlight.slug = user.slug
WHERE
article.slug IS NOT NULL OR user.slug IS NOT NULL
Another example of doing this:
SELECT `highlight`.`slug` from `highlight`
WHERE `highlight`.`slug` IN (SELECT `user`.`slug` FROM `user` UNION SELECT `article`.`slug` FROM `article`)
OR
SELECT `highlight`.`slug` from `highlight`
INNER JOIN (SELECT `user`.`slug` FROM `user` UNION SELECT `article`.`slug` FROM `article`) AS `allslugs` ON `highlight`.`slug` = `allslugs`.`slug`
Another update, I call this one "fun with joins"
SELECT `highlight`.`slug` from `highlight`
RIGHT JOIN `user` ON `highlight`.`slug` = `user`.`slug`
LEFT JOIN `article` ON `highlight`.`slug` = `article`.`slug`
WHERE
`highlight`.`slug` IS NOT NULL

Answer 2

Try changing your query to qualify the column name in select list
SELECT h.`slug` from HighlightTable h
INNER JOIN ArticleTable a ON h.`slug` = a.`slug`
INNER JOIN UserTable u ON h.`slug` = u.`slug`;
Can't reproduce the issue. See This Fiddle
per your latest comment you need a LEFT JOIN query like
SELECT h.`slug` from HighlightTable h
LEFT JOIN ArticleTable a ON h.`slug` = a.`slug`
LEFT JOIN UserTable u ON h.`slug` = u.`slug`;
Then do a separate JOIN and UNION the result set
SELECT h.`slug` from HighlightTable h
INNER JOIN ArticleTable a ON h.`slug` = a.`slug`
UNION
SELECT h.`slug` from HighlightTable h
INNER JOIN UserTable u ON h.`slug` = u.`slug`;