$uid = $_GET['uid'];
if ($uid) { // 很机智
    vendor("Func.Func");
    if (Func::checkMobile($uid)) {
        $uid = $user->where(['telphone' => $uid])->getField('id');
    }
    $where['uid'] = $uid;
    $this->assign('uid', $_GET['uid']);
    $tail .= '&uid=' . $_GET['uid'];
}

$act_name   = trim($_GET['act_name']);
$groupModel = M('group_purchase');
if ($act_name) {
    $this->assign('act_name', $act_name);
    $tail       .= '&act_name=' . $act_name;
    $group_flag = $groupModel->where(['title' => ['like', '%' . $act_name . '%']])->field('id')->select();

    $group_ids = [];
    foreach ($group_flag as $v) {
        $group_ids[] = $v['id'];
    }

    $where['group_id'] = ['in', $group_ids];
}

$group_id = trim($_GET['group_id']);
if ($group_id) {
    $this->assign('group_id', $group_id);
    $tail                  .= '&group_id=' . $group_id;
    $where['group_id'] = $group_id;
}

这段查询，很机智。
灵活判断，分别处理。
直接查不到，那就间接查询。