node连续查询两次数据库返回方式（文档未定）

function db(callback){

var mysql  = require('mysql');  

var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'root',
  password : '',
  port: '3306',
  database: 'test',
}); 

connection.connect();

var  sql = 'SELECT * FROM uer';
var  sql1 = 'SELECT * FROM member';
//查
var rr;
var tt;
connection.query(sql,function (err, result) {
        if(err){
          console.log('[SELECT ERROR] - ',err.message);
          return;
        }

      rr=JSON.stringify(result)
      //callback(rr);
});
connection.query(sql1,function (err, result) {
        if(err){
          console.log('[SELECT ERROR] - ',err.message);
          return;
        }

      tt=JSON.stringify(result)
      if (rr && tt) {
      	 callback(rr,tt);
      }

});
/*connection.query(sql,function (err, result) {
        if(err){
          console.log('[SELECT ERROR] - ',err.message);
          return;
        }
  var r=2;
      json=JSON.stringify(result)
      callback(json);
});*/

connection.end();
}
exports.db=db;
var a;
var b;
var c;
/*function callback1(type,result){

    c[type]=result;

    if (c[1] && c[2]) {

    	res.end(c);

    }

}*/