javascript - 如何重复执行nodejs mysql查询直到得到期望的结果？

Question

conn.connect(function (err) {
        if (err) throw err;
        var prizeCode = generatePrizeCode();// this method return a 5-digit code
        var query = "SELECT * FROM user WHERE code = ? ";
        var values = [[prizeCode]];
        conn.query(query, [values], function (err, result) {
            if (err) throw err;
            // Here I want to re-execute the above query if the result is not empty 
        });
    });

在上面的代码中，只要结果包含数据，我就想执行查询。但我不能像下面的伪代码那样使用循环，

// if the result is empty, the generated code does not exist in database. So it can be used.
while(result.length != 0){
    var result = conn.query("SELECT * FROM user WHERE code = abc");
}

我怎样才能做到这一点？

标签： javascriptmysqlnode.jsdatabasenode-mysql

Answer 1

您必须使用回调或承诺/异步函数。以下是如何使用回调编写它：

const mysql = require('mysql');

const retryLimit = 50;

const connection = mysql.createConnection({
  database: 'test',
  host: 'localhost',
  password: 'hunter2',
  user: 'dave',
});

function getData(attempts, cb) {
  if (attempts < retryLimit) { // we haven't exausted our attempts yet
    const code = generatePrizeCode();
    connection.query('SELECT * FROM user WHERE code = ?', code, (err, result) => {
      if (err) {
        return cb(err);
      }
      if (result.length > 0) { // code already exists
        getData(attempts + 1, cb); // recurse
      } else { // this is a new code
        cb(null, code); // return the new code via the callback function
      }
    });
  } else { // we have exausted our attempts
    return cb(new Error('retry limit exceeded'));
  }
}

getData(0, (err, code) => {
  // do what you want here, e.g., console.log(code)
  connection.end();
});

请注意，如果您递归太多次并超过最大调用堆栈，这可能会导致节点崩溃。一种解决方法是调用 setImmediate 或 setTimeout 而不是直接递归。而不是线

getData(attempts + 1, cb); // recurse

利用

setImmediate(() => {
  getData(attempts + 1, cb); // recurse
});

或者

// recurse, using setImmediate every 1000th time
if (attempts % 1000 === 0) {
  setImmediate(() => {
    getData(attempts + 1, cb); // recurse
  });
} else {
  getData(attempts + 1, cb); // recurse
}

使用 promises 和 async/await 风格会清理很多，并且可能看起来更像你习惯的（注意这使用了promise-mysql库）：

const mysql = require('promise-mysql');

async function getData(connection) {
  const retryLimit = 50;
  for (let i = 0; i < retryLimit; i++) {
    const code = generatePrizeCode();
    const result = await connection.query('SELECT * FROM user WHERE code = ?', code);
    if (result.length === 0) { // this code doesn't exist yet
      return code;
    }
  }
  throw new Error('retry limit exceeded');
}

(async () => {
  try {
    const connection = await mysql.createConnection({
      database: 'test',
      host: 'localhost',
      password: 'hunter2',
      user: 'dave',
    });
    try {
      const code = await getData(connection);
      // do what you want here, e.g., console.log(code)
    } finally {
      connection.end();
    }
  } catch (err) {
    console.log(err);
  }
})();