在使用的promise的时候，我有希望在then之后再执行一个promise，但是多次执行之后就有很深的嵌套了，如下文的register，感觉没有抓住Promise的精髓，请大大们指导

//dboperation
exports.getPool = function(sql){
	return new Promise(function(resolve,reject){
		pool.getConnection(function(err,connection){
			if(err){
				reject('error connecting: ' + err.stack);
			}else{
				connection.query(sql,function(Err,Rows){
					if(Err){
						reject('error connecting: ' + Err.stack);
					}
					connection.release();
					resolve(Rows)
				});
			}
		})
	});
}	

//server
function register(socket,username, password, petname){
    var REGISTER = 0,
        p1 = dbobj.getPool("select * from user_table where Username='"+username+"'");
    p1.then(function(data){
        console.log(data);
        if(data.length === 1){
            REGISTER = -1;
            socket.emit('REGIST_RESULT', { REGISTER: REGISTER});
        }else{
            var p2 = dbobj.getPool("select count(*) from user_table");
            p2.then(function(data){
                var id = data[0]["count(*)"] + 1,
                    sql = "insert into user_table (id,username,password,petname,avatar) values(?,?,?,?,?)",
                    inserts = [
                    id,
                    username, 
                    password, 
                    petname,
                    null
                    ];
                sql = mysql.format(sql,inserts);
                var p3 = dbobj.getPool(sql);
                p3.then(function(data){
                    var p4 = dbobj.getPool("select * from user_table where Username='"+username+"'");
                    p4.then(function(data){
                        console.log(data);
                        if(data.length === 1){
                            REGISTER = 1;
                            socket.emit('REGIST_RESULT', { REGISTER: REGISTER});
                        }else{
                            REGISTER = "error";
                            socket.emit('REGIST_RESULT', { REGISTER: REGISTER});
                        }
                    })
                    .catch(function(err){
                        console.log("insert into user_table"+err)
                        socket.emit('REGIST_RESULT', { REGISTER: REGISTER});
                    })

                })
                .catch(function(err){
                    console.log("insert into user_table"+err)
                    socket.emit('REGIST_RESULT', { REGISTER: REGISTER});
                })
            })
            .catch(function(err){
                console.log("select count(*) from user_table"+err)
                socket.emit('REGIST_RESULT', { REGISTER: REGISTER});
            });
        }
    })
    .catch(function(err){
        console.log("select * from user_table"+err)
        socket.emit('REGIST_RESULT', { REGISTER: REGISTER});
    }); 
}

io.on('connection', function (socket) {
  socket.emit('news', { hello: 'world' });
  socket.on('REGISTER', function (username, password, petname) {
      console.log(new Date());  
      register(socket,username, password, petname);
  }) 
});

//client
socket.emit('REGISTER',"test", "zxcz", "petname");

socket.on('news', function (data) {
    console.log(data);
})

socket.on("REGIST_RESULT",function(data){
    console.log(data);
})