PHP - 使用MAX（）的SQL查询返回null

Question

我试图检索已插入数据库的记录。我使用MAX(id)执行此操作，但它始终返回null。我已尝试在phpmyadmin中运行查询，但它确实有效。我已尝试过我在网站上找到的其他方法，但都没有。非常感谢帮助。

我的代码：

    $pdo = Database::connect();
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $sql = "INSERT INTO tblgroups (cid, name) values(?, ?)";
    $q = $pdo->prepare($sql);
    $q->execute(array("1", $name));

    $sql = "SELECT MAX(gid) AS gid FROM tblgroups";
    $q = $pdo->prepare($sql);
    $data = $q->fetch(PDO::FETCH_ASSOC);

    $gid = $data['gid'];

    $sql = "INSERT INTO tblcategories (cid, gid, name, text_color, bg_color) values(?, ?, ?, ?, ?)";
    $q = $pdo->prepare($sql);
    $q->execute(array("1", $gid, $name, "#FF0000", "#FFFFFF"));

    $sql = "INSERT INTO tblusers (username, password, gid) values(?, ?, ?)";
    $q = $pdo->prepare($sql);
    $q->execute(array($name, md5("password"), $gid));


    $sql = "SELECT MAX(uid) AS uid FROM tblusers";
    $q = $pdo->prepare($sql);
    $data = $q->fetch(PDO::FETCH_ASSOC);

    $uid = $data['uid'];


    $sql = "INSERT INTO tbluser_groups (gid, uid) values(?, ?)";
    $q = $pdo->prepare($sql);
    $q->execute(array($gid, $uid));