单元测试以数据库为中心的网站

I'm using a legacy (no-tests) PHP project, full of classes like http://pastebin.com/byBMTAEY

Practically all methods are static, and most of them reference the database directly using a singleton (to limit connections to one)

Most of them also have a reference to a global called _COMPANY, that identifies which company is using the platform (based on the subdomain).

My (very newbie) question is: what's the best way to unit test this class? Should I unit test it at all, since what it does is go to the database and do logic there, and then just wrap up the result?

I mean, I can probably mock the database class, and then say "on 'one' return this result-object", but then what am I testing really? The SQL logic not, for one thing, only the private functions to make a business object... is that enough for the unit test? Or should I somehow also unit test the sql? Or is that reserved for integration testing?

Please help me understand so I can use this practice if applicable.

Example code also included here for reference:

<?php

require_once('DataBase.php');
require_once('businessobjects/User.php');

class UserMgt {

    public static function login($email, $pass){
        $db = DataBase::getInstance();

        $sql = "
            SELECT * 
            FROM user
            WHERE login = ? AND password = ? AND active = 1 AND company_id = ?
            ";
        $data = array(
            $email, md5($pass), $GLOBALS['_COMPANY']->id,
        );

        $user = null;
        if ($row = $db->one($sql, $data)) { // fetch only one row using the query and parameters
            $user = self::copyToUserObject($row); // make a business object
            $user->setCapabilities(self::getUserCapabilities($user->getID())); // perform some extra hydration of business object
        }
        return $user;
    }

    private static function copyToUserObject($dbrow) {
        $user = new User();
        // do stuff
        return $user;
    }

    private static function getUserCapabilities($userid) {
        // do sql query stuff
        // return array
    }
}