Meta query and compare “!=” not working as expected

I’ve found the solution (I suppose): looks like that the right operator in this case is “NOT EXISTS”. I’ve noticed that in the db postmeta table I’ve got only the records of the “is_hidden” key with value “on”…

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)