Question

how can insert a record if only a specific value doesn’t exist in a field

  INSERT INTO `users` (fb_id, username) 
    SELECT '.$id_signed_in.', '.$user_signed_in.' FROM `users` 
    WHERE  NOT EXISTS (SELECT * FROM `users` 
                      WHERE fb_id='.$id_signed_in.') 
    LIMIT 1'

this does not seem to work

Answer 1

Please try this ,

INSERT INTO `users` (fb_id, username) 
SELECT '.$id_signed_in.', '.$user_signed_in.' FROM `users` 
WHERE fb_id NOT IN(SELECT fb_id FROM `users` 
                  WHERE fb_id='.$id_signed_in.') 
LIMIT 1'