Vypadá to jako trochu hackovací práce, ale podařilo se mi to zprovoznit pomocí funkce LAST_INSERT_ID() zabudované do MySQL.
CREATE TRIGGER `order_guide_insert_trigger`
BEFORE INSERT ON `order_guide`
FOR EACH ROW
BEGIN
IF NEW.Sort_Placement IS NULL THEN
SET NEW.Sort_Placement = LAST_INSERT_ID() + 1;
END IF;
END;
To také funguje a zdá se, že funguje
CREATE TRIGGER `order_guide_insert_trigger`
BEFORE INSERT ON `order_guide`
FOR EACH ROW
BEGIN
IF NEW.Sort_Placement IS NULL THEN
SET NEW.Sort_Placement = (SELECT ID FROM order_Guide ORDER BY id DESC LIMIT 1) + 1;
END IF;
END;