core/DB.php
public static function getOne(string $sql, $args = null, $field = null)
    {
        return self::engine()->getOne($sql, $args, $field);
    }
public static function getOne(string $sql, $args = null, $field = null)
    {
        return self::engine()->getOne($sql, $args, $field);
    }
获取单个字段值
@param string $sql
@param null $args
@param null $field
@return mixed|null
@throws MysqlException