MYSQL QUERY ERROR

Query: SELECT ft.*, ( SELECT id FROM mb_files WHERE files_tree_id = ft.id AND site_id = "1" LIMIT 1 ) AS files_exist, ( SELECT id FROM mb_files_tree WHERE files_tree_id = ft.id AND site_id = "1" AND view = 1 ) AS tree_exist FROM mb_files_tree ft WHERE files_tree_id = "0" AND site_id = "1" AND view = 1 ORDER BY sort
Error: Subquery returns more than 1 row