core/DB.php
public static function getMax(string $tbname, string $field, $where = null, $args = null)
    {
        return self::engine()->getMax($tbname, $field, $where, $args);
    }
public static function getMax(string $tbname, string $field, $where = null, $args = null)
    {
        return self::engine()->getMax($tbname, $field, $where, $args);
    }
获取某个字段的最大值
@param string $tbname
@param string $field
@param null $where
@param null $args
@return null
@throws MysqlException