如何避免 MySQL 中重复的随机 id 选择？

Question

我需要从表nodes 中选择两个唯一的随机id 并将其插入edges 表的parent 列中。 但问题是它们有时会重复，并且需要它们始终不同，请问我该如何避免这种情况？

代码：

const mysql = require('mysql');

const connection = mysql.createConnection({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'DAGtest3'
});

connection.connect((err) => {
  if (err) throw err;
  console.log('Database Connected!');
});

var a = "INSERT INTO `edges` (`parent`, `child`) VALUES ((SELECT `id` FROM `nodes` ORDER BY rand() LIMIT 1 ) , (4) )";

connection.query(a, (err, res) => {
    if(err) throw err;
    console.log("1 edge inserted to previous data");
});
            
var b = "INSERT INTO `edges` (`parent`, `child`) VALUES ((SELECT `id` FROM `nodes` ORDER BY rand() LIMIT 1) , (4) )";
connection.query(b, (err, res) => {
    if(err) throw err;
    console.log("Another edge inserted to previous data");   
});

表格代码：

CREATE TABLE nodes (
    id INTEGER NOT NULL AUTO_INCREMENT,
    sensorvalue VARCHAR(255) NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    PRIMARY KEY (id)
);
 
CREATE TABLE edges (
    parent INTEGER NOT NULL REFERENCES nodes(id) ON UPDATE CASCADE ON DELETE CASCADE,
    child INTEGER NOT NULL REFERENCES nodes(id) ON UPDATE CASCADE ON DELETE CASCADE,
    PRIMARY KEY (parent, child)
);
 
CREATE INDEX parent_idx ON edges (parent);
CREATE INDEX child_idx ON edges (child);

提前致谢。

Answer 1

您不想要两个连续的随机选择，因为第二个随机选择返回与第一个相同的行的可能性非零。我认为您应该一次获取两行：

INSERT INTO `edges` (`parent`, `child`) SELECT `id`, 4 FROM `nodes` ORDER BY rand() LIMIT 2;

因此，您的代码将只执行一个查询，而不是两个：

var a = "INSERT INTO `edges` (`parent`, `child`) SELECT `id`, 4 FROM `nodes` ORDER BY rand() LIMIT 2";

connection.query(a, (err, res) => {
    if(err) throw err;
    console.log("2 edges inserted to previous data");
});

当我在本地设置你的数据库并运行上面的代码时，我得到了一个成功的插入：

Database Connected!
2 edges inserted to previous data

当我在控制台中查询数据库时，我得到：

mysql> select * from edges;
+--------+-------+
| parent | child |
+--------+-------+
|      2 |     4 |
|      3 |     4 |
+--------+-------+
2 rows in set (0.00 sec)