Question

I am getting Unknown column 'u.id' in 'on clause'.

SELECT id, username,
    coalesce(
     (SELECT name from company c 
        INNER JOIN user_company uc 
        ON uc.user_id = u.id 
        AND c.id = uc.company_id), 'NOT-AVAILABLE' 
    ) companyname
FROM `user` u

Answer 1

Based on your comment, the correlation can’t be placed within the JOIN of the correlated sub-query.

It can, however, be placed in the WHERE clause of the correlated sub-query.

SELECT
  id,
  username,
  coalesce(
    (SELECT name
       FROM company      c
 INNER JOIN user_company uc
         ON c.id = uc.company_id
      WHERE uc.user_id = u.id
    ),
    'NOT-AVAILABLE'
  ) companyname
FROM
  `user` u 

That answers your explicit question; why your query failed syntactically.

I would, however, replace the whole correlation with a simple LEFT JOIN.

SELECT
  u.id,
  u.username,
  COALESCE(c.name, 'NOT-AVAILABLE') companyname
FROM
  `user` u 
LEFT JOIN
  `user_company` uc
      ON uc.user_id = u.id
LEFT JOIN
  `company` c
    ON c.id = uc.company_id