我尝试在mysql中随机从查找表中选择名字来构建测试数据集。我有一个表，有200个名字，性别和行id从1到200。像这样：

id   firstname   gender
1    Aaron       m
2    Adam        m
3    Alan        m
etc...

我使用随机生成器从该表中选择以下查询：

SELECT id, firstname FROM firstname WHERE id = round(1 + (rand() * 199));

我希望随机数与查找表中的一个id相加，从而生成一个结果，如

id   firstname
43   Jason

一次又一次地运行代码会给我一个
单行（同上）
或多行，如 id firstname 29 Ethan 147 Jean 或者没有结果（两个字段都为空）。
如果我自己运行随机生成器，它将始终生成1到200之间的数字。正如您在下面看到的，id字段是int，如果将结果强制转换为signed，查询的行为与此相同。我也试过用地板代替圆形，只是想看看效果是否有什么不同——唉，没有。
有人能告诉我为什么会出现这种异常吗？我错过了什么？
以下是一些代码，用于创建原始表的前20行以进行测试：

-- First Name --
drop table if exists firstname;

  CREATE TABLE firstname (
  id INT NOT NULL,
  firstname VARCHAR(20) NOT NULL,
  gender VARCHAR(1) NOT NULL,
  PRIMARY KEY (id),
  UNIQUE (firstname)
  );

  INSERT INTO firstname
  (id,firstname,gender)
  VALUES
  (1,"Aaron","m"),
(2,"Adam","m"),
(3,"Alan","m"),
(4,"Albert","m"),
(5,"Alexander","m"),
(6,"Andrew","m"),
(7,"Anthony","m"),
(8,"Arthur","m"),
(9,"Austin","m"),
(10,"Benjamin","m"),
(11,"Billy","m"),
(12,"Bobby","m"),
(13,"Brandon","m"),
(14,"Brian","m"),
(15,"Bruce","m"),
(16,"Bryan","m"),
(17,"Carl","m"),
(18,"Charles","m"),
(19,"Christian","m"),
(20,"Christopher","m");