问题
Am sure this is fairly obvious, to me it would make more sense if the second double question mark in the example was a single one.
From their docs:
Alternatively, you can use ?? characters as placeholders for identifiers you would like to have escaped like this:
var userId = 1;
var columns = ['username', 'email'];
var query = connection.query('SELECT ?? FROM ?? WHERE id = ?', [columns, 'users', userId], function(err, results) {
// ...
});
console.log(query.sql); // SELECT `username`, `email` FROM `users` WHERE id = 1
回答1:
?? is used for table and column names, it escapes them with backticks. ? is for ordinary values.
来源：https://stackoverflow.com/questions/30457086/what-is-the-difference-between-single-and-double-question-mark-in-n