为什么第一个程序起作用，而第二个程序却不起作用？

Question

var mysql = require('mysql')

var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'user',
  password : 'pass',
  database : 'mydb'
})

var values = [ ['Dave', 20],
                    ['Judd', 22],
                    ['Snek', 24],
                    ['Dale', 26] 
            ];

connection.connect(function(err) {
  if (err) {
    return console.error('error: ' + err.message);
  }

  let createTodos = `create table if not exists siblings(
                          id int primary key auto_increment,
                          name varchar(255)not null,
                          age tinyint(2) not null default 0
                      )`;

  connection.query(createTodos, function(err, results, fields) {
    if (err) {
      console.log(err.message);
    }
  });

  var sql = 'insert into siblings (name, age) values ?';
connection.query(sql, [values], function (err, result) {
    if (err) throw err;
    console.log("Number of records inserted: " + result.affectedRows);
  });
});

This works perfectly fine and does what I intended, creates the table with column for name and age if the table does not already exist, then inserts the values in the value array accordingly.

This does not work

var mysql = require('mysql')

var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'user',
  password : 'pass',
  database : 'mydb'
})

var values = [ ['Dave', 20],
                    ['Judd', 22],
                    ['Snek', 24],
                    ['Dale', 26] 
            ];

connection.connect(function(err) {
  if (err) {
    return console.error('error: ' + err.message);
  }

  let createTodos = `create table if not exists siblings(
                          id int primary key auto_increment,
                          name varchar(255)not null,
                          age tinyint(2) not null default 0
                      )`;

  connection.query(createTodos, function(err, results, fields) {
    if (err) {
      console.log(err.message);
    }
  });

});

var sql = 'insert into siblings (name, age) values ?';
    connection.query(sql, [values], function (err, result) {
        if (err) throw err;
        console.log("Number of records inserted: " + result.affectedRows);
      });

The difference is that the query to insert the values array is outside of the connect function call. I understand that this must have something to do with the callback functions being passed to connect and them perhaps being async functions, but can someone correct me or elaborate further on this? The control flow of node is eluding me.

Answer 1

You need to connect to the database before you can execute queries on the database.

connection.connect() is an asynchronous function, which means that when you call it, it begins connecting to the database and returns immediately. Your code continues on, and when you try and execute a query, the database has not yet connected to the database yet.

This is why it's important to execute your queries in the connection callback. When your database is connected, the callback allows your code to continue when the database has successfully (Or unsuccessfully) finishes connecting.

I highly suggest reading into asynchronous programming, and look into promises/await syntax. They essentially allow your code to block, by chaining everything neatly instead of involving many many layers of callbacks.

The promise-mysql package makes this dirt simple for you:

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

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

const values = [
  ['Dave', 20],
  ['Judd', 22],
  ['Snek', 24],
  ['Dale', 26] 
];

async main() {
  const dbErr = await connection.connect();
  if (dbErr) {
    return console.error('error: ' + dbErr.message);
  }

  {
    const createTodos = `create table if not exists siblings(
      id int primary key auto_increment,
      name varchar(255)not null,
      age tinyint(2) not null default 0
    )`;
    const [err, results, fields] = await connection.query(createTodos);
  }

  {
    const insertSiblings = 'insert into siblings (name, age) values ?';
    const [err, results, fields] = await connection.query(insertSiblings);

    console.log("Number of records inserted: " + result.affectedRows);
  }
}

// Call the async function
main();

Answer 2

The issue is that presumably the connect operation takes some time to complete. In the first sample, because the subsequent calls are included in the callback, they happen only after the connection is established to the database.

In the second case, because the query is outside the callback to connect, the query is attempted before the connection is established so it fails.

Even in the first example, you have the same mistake between the call to create the table and the subsequent call to query it. But most likely the call to create the table happens so quickly that in most cases it will just work but you are bound to see failures every now and then.

The correct way to write this sequence, using callbacks, is to chain them like so:

var mysql = require('mysql')

var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'user',
  password : 'pass',
  database : 'mydb'
})

var values = [ ['Dave', 20],
                    ['Judd', 22],
                    ['Snek', 24],
                    ['Dale', 26] 
            ];

connection.connect(function(err) {
  if (err) {
    return console.error('error: ' + err.message);
  }

  // this happens only after the connect operation is complete
  let createTodos = `create table if not exists siblings(
                          id int primary key auto_increment,
                          name varchar(255)not null,
                          age tinyint(2) not null default 0
                      )`;

  connection.query(createTodos, function(err, results, fields) {
    if (err) {
      console.log(err.message);
    }

    // this happens only after the connect & create table are complete
    var sql = 'insert into siblings (name, age) values ?';
    connection.query(sql, [values], function (err, result) {
      if (err) throw err;
      // this happens only after connect, create & insert are complete
      console.log("Number of records inserted: " + result.affectedRows);
    });
  });
});

Answer 3

On the second code you put your query outside connection.connect(function(err) {

In the first one it is inside.