drop procedure if EXISTS autodecrement;
delimiter $$
CREATE PROCEDURE autodecrement(IN tableName VARCHAR(50),IN fieldName VARCHAR(20),IN id int,IN action VARCHAR(10),IN step int,OUT result int)
BEGIN
  set @sql = concat('update ',tableName,' set ',fieldName,' = ',fieldName,action,step,' where id = ',id,' and ',fieldName,action,step,' > 0');
  prepare stmt from @sql;
  execute stmt;
  select ROW_COUNT() into result;
END$$
delimiter;