Get data from MySQL database and pass it to jade an access it using nodejs

I have started nodejs to make a simple chat application in it. For this I want to access all users which are in mysql database.

My code is like:

JS

exports.authenticate = function(req, res) {

    //connection.connect();
    var sql="SELECT * from users where username='"+req.body.user+"' and password='"+req.body.pass+"' LIMIT 1";
    connection.query(sql, function(err, rows, fields) {
        if (err) throw err;

        //res.send('Your data is: ', rows);
        var str="Hi, <b>"+rows[0].name+"</b> ("+rows[0].email+")";

        sql="SELECT DISTINCT username,name from users ORDER BY name";
        connection.query(sql, function(err, datarows, fields) {
            if (err) throw err;
            //res.send('Your data is: ', rows+' <br/> All Users are : ', datarows.length+"<a href='/'>Login</a>");
            str+='<ul style="list-style:none;width:300px">';
            for(var index=0;index<datarows.length;index++)
            {
                str+="<li><a href='javascript:;'>"+datarows[index].name+", "+datarows[index].email+"</a></li>";
            }
            str+='</ul>';console.log(str);  
            console.log(str)//gives ul  
        });

        str+="<a href='/'>Login</a>";
        res.send(str);//this not gives the ul of other users
    });

}

The above code has problems that I wrote console.log(str)//gives ul this prints the whole string like Hi, <b>Rohan</b> ("rohan@xyz.com")<ul><li><a>Something</a></li>....</ul>. But res.send(str); sends only Hi, <b>Rohan</b> ("rohan@xyz.com")<a href='/'>Login</a>.

1. Why this is happening?
2. Is my str variable not global?
3. Can I use res.send() many times, if yes then how?

Can I use the above code with jade then what code should I write for this.

I found this answer How to pass data form mongodb into nodejs view (jade)? related to my problem

app.get('/', function(req, res) {
  mgs(dbColl).find(function(data){
    res.render('yourview.jade', { data: data });
  });
});

Then,How to access data in yourview.jade

I also want to know for which type of applications we should use nodejs ?

Thank in advance.