How MySQL query works with and without placeholders (?)?

Question

I have two approaches to execute sql query in node.js , I want to know difference between these two. Which one is recommended to use ? and why ??

First Approach

var userId = 1;
var columns = ['username', 'email'];    
var query = connection.query('SELECT ?? FROM ?? WHERE id = ?', 
  [columns, 'users', userId], function(err, results) 
{   
  // ... 
});

Second Approach

var userId = 1;
var query = connection.query('SELECT username,email FROM UserTable WHERE id=?', 
  [userId], function(err, results) 
{   
  // ... 
});

Please tell me exact use of these two mechanisms.

Answer 1

The double question mark syntax ?? in the substitution string wraps the substituted parameters in delimiters before sending them to the server. The delimiters are appropriate for table and column names. So your first example, 'SELECT ?? FROM ?? WHERE id = ?' sends

SELECT `username`, `email` FROM `users` WHERE id = '1'

to the server.

The second one, 'SELECT username,email FROM UserTable WHERE id=?', sends

SELECT username,email FROM UserTable WHERE id='1'