How to get the results from nodejs using mysql package?

first, i connect the db and select DB:

var defaultOptions = {
    user: "root",
    pwd:'admin',
    db:"britcham_dev_local",
    server:"local", // Maybe we don't need this variable.
};

var client = new Client();
client.user = defaultOptions.user;
client.password = defaultOptions.pwd;
client.connect(function (error, results) {
   // 
});
client.query('USE ' + defaultOptions.db, function (error, results) {
   // 
});

Second, I query with client object:

var self = this;
var this.users;
client.query("SELECT * FROM users", function (error, results, fields) {
    if (error) {
        //
    }
    if (results.length  开发者_开发知识库> 0) {
        self.users = results;
    }
});

console.log(this.users);

it's nothing output ??? Why ??

Since node.js is non-blocking and asynchronous, then in this code:

client.query("SELECT * FROM users", function (error, results, fields) {
    if (error) {
        //
    }
    if (results.length  > 0) {
        self.users = results;
    }
});

console.log(this.users);

data from DB are not probably loaded yet into users variable when you are trying to log it into console. You can check it out if you do your console.log operation within the query, for example:

client.query("SELECT * FROM users", function (error, results, fields) {
    if (error) {
        //
    }
    if (results.length  > 0) {
        console.log(results);
    }
});

To pass the result into a variable when the operation is finished you can wrap your client DB call into a function with callback parameter and set your variable when the callback is invoked, for example:

function query(sql, callback) {
    client.query(sql, function (error, results, fields) {
        if (error) {
            //
        }
        if (results.length  > 0) {
            callback(results);
        }
    });
}

query("SELECT * FROM users", function(results) {
    self.users = results;
    console.log(self.users);
});

Above code is just a concept.

How is the suggested answer different from this?

var self = this;
var this.users;
client.query("SELECT * FROM users", function (error, results, fields) {
    if (error) {
    //
    }
    if (results.length  > 0) {
        self.users = results;
        console.log(this.users);
    }
});

I might be wrong this is not different from the suggested answer in that it writes to console no sooner than when we have the data back from the DB.

The suggested answer seems only to add yet another function?