谢谢，感谢所有的帮助！我对mysql相当陌生。这里是参数。

string query = String.Format("UPDATE buildings JOIN server_buildings ON buildings.global_id = server_buildings.global_id AND buildings.level = server_buildings.level SET buildings.storage = buildings.storage + (server_buildings.speed * ?Num * IF(buildings.boost >= NOW(), 2, 1)) WHERE buildings.global_id IN('generator', 'test')");
                     
            using (MySqlCommand command = new MySqlCommand(query, connection))
            {
                MySqlParameter param = new MySqlParameter("?Num", (deltaTime / 3600d));
                command.Parameters.Add(param);
      
               command.ExecuteNonQuery();
            }

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找到解决办法了。我的系统语言不是英语。解决方案为：

(deltaTime / 3600d).ToString("F7").Replace(",", ".")

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