water/server/plugin/saiadmin/app/service/device/DeviceStatusPolicy.php

<?php
declare(strict_types=1);

namespace plugin\saiadmin\app\service\device;

/** One mutually exclusive display policy for counters, filters, cards and exports. */
final class DeviceStatusPolicy
{
    public const LABELS = [
        'online' => '在线',
        'offline' => '离线',
        'fault' => '告警',
        'unconnected' => '未接入',
    ];

    public function __construct(private readonly \DateTimeImmutable $now)
    {
    }

    public function expression(): string
    {
        $cutoff = $this->now->modify('-15 minutes')->format('Y-m-d H:i:s');
        return "CASE
            WHEN TRIM(COALESCE(d.fault, '')) <> '' THEN 'fault'
            WHEN d.last_seen IS NULL THEN 'unconnected'
            WHEN d.last_seen < '$cutoff' THEN 'offline'
            ELSE 'online' END";
    }
}