sqlqueries


The XMLHttpRequest Object

The XMLHttpRequest object is used to exchange data with a server behind the scenes.

This means that it is possible to update parts of a web page, without reloading the whole page.

Sending asynchronous requests is a huge improvement for web developers. Many of the tasks performed on the server are very time consuming.

Before AJAX, this operation could cause the application to hang or stop.

With AJAX, the JavaScript does not have to wait for the server response, but can instead:
  • execute other scripts while waiting for server response
  • deal with the response when the response ready


Create an XMLHttpRequest Object

All modern browsers (IE7+, Firefox, Chrome, Safari, and Opera) have a built-in XMLHttpRequest object.
Syntax for creating an XMLHttpRequest object:
variable=new XMLHttpRequest();


 Old versions of Internet Explorer (IE5 and IE6) uses an ActiveX Object:
variable=new ActiveXObject("MicrosoftXMLHTTP");


 To handle all modern browsers, including IE5 and IE6, check if the browser supports the XMLHttpRequest object.
If it does, create an XMLHttpRequest object, if not, create an ActiveXObject:

var xmlhttp;

if (window.XMLHttpRequest)  {// code for IE7+, Firefox, Chrome, Opera, Safari  xmlhttp=new XMLHttpRequest();  }
else  {// code for IE6, IE5  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");  }

Send a Request To a Server

To send a request to a server, we use the open() and send() methods of the XMLHttpRequest object:

xmlhttp.open("GET","ajax_info.txt",true);

xmlhttp.send();


Async=true
When using async=true, specify a function to execute when the response is ready in the onreadystatechange event:
<html>
<head>
<script>
function loadXMLDoc()
{
var xmlhttp;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","ajax_info.txt",true);
xmlhttp.send();
}
</script>
</head>
<body>

<div id="myDiv"><h2>Let AJAX change this text</h2></div>
<button type="button" onclick="loadXMLDoc()">Change Content</button>

</body>
</html>

Async=false

To use async=false, change the third parameter in the open() method to false:

xmlhttp.open("GET","ajax_info.txt",false);

Using async=false is not recommended, but for a few small requests this can be ok.

Remember that the JavaScript will NOT continue to execute, until the server response is ready. If the server is busy or slow, the application will hang or stop.

Note: When you use async=false, do NOT write an onreadystatechange function - just put the code after the send() statement:
<html>
<head>
<script>
function loadXMLDoc()
{
var xmlhttp;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.open("GET","ajax_info.txt",false);
xmlhttp.send();
document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
}
</script>
</head>
<body>

<div id="myDiv"><h2>Let AJAX change this text</h2></div>
<button type="button" onclick="loadXMLDoc()">Change Content</button>

</body>
</html>

Server Response

To get the response from a server, use the responseText or responseXML property of the XMLHttpRequest object.

PropertyDescription
responseTextget the response data as a string
responseXMLget the response data as XML data


The responseText Property

If the response from the server is not XML, use the responseText property.
The responseText property returns the response as a string, and you can use it accordingly:

<html>
<head>
<script>
function loadXMLDoc()
{
var xmlhttp;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","ajax_info.txt",true);
xmlhttp.send();
}
</script>
</head>
<body>

<div id="myDiv"><h2>Let AJAX change this text</h2></div>
<button type="button" onclick="loadXMLDoc()">Change Content</button>

</body>
</html>

The responseXML Property

If the response from the server is XML, and you want to parse it as an XML object, use the responseXML property:


<html>
<head>
<script>
function loadXMLDoc()
{
var xmlhttp;
var txt,x,i;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    xmlDoc=xmlhttp.responseXML;
    txt="";
    x=xmlDoc.getElementsByTagName("ARTIST");
    for (i=0;i<x.length;i++)
      {
      txt=txt + x[i].childNodes[0].nodeValue + "<br>";
      }
    document.getElementById("myDiv").innerHTML=txt;
    }
  }
xmlhttp.open("GET","cd_catalog.xml",true);
xmlhttp.send();
}
</script>
</head>

<body>

<h2>My CD Collection:</h2>
<div id="myDiv"></div>
<button type="button" onclick="loadXMLDoc()">Get my CD collection</button>
 
</body>
</html>


The onreadystatechange event
When a request to a server is sent, we want to perform some actions based on the response.

The onreadystatechange event is triggered every time the readyState changes.

The readyState property holds the status of the XMLHttpRequest.

Three important properties of the XMLHttpRequest object:

PropertyDescription
onreadystatechangeStores a function (or the name of a function) to be called automatically each time the readyState property changes
readyStateHolds the status of the XMLHttpRequest. Changes from 0 to 4:
0: request not initialized
1: server connection established
2: request received
3: processing request
4: request finished and response is ready
status200: "OK"
404: Page not found

In the onreadystatechange event, we specify what will happen when the server response is ready to be processed.

When readyState is 4 and status is 200, the response is ready:


Note: The onreadystatechange event is triggered five times (0-4), one time for each change in readyState.









SQL Queries

Finding the nth highest salary of an employee.

Create a table named Employee_Test and insert some test data as:-
CREATE TABLE Employee_Test
(
Emp_ID INT Identity,
Emp_name Varchar(100),
Emp_Sal Decimal (10,2)
)

INSERT INTO Employee_Test VALUES ('Anees',1000);
INSERT INTO Employee_Test VALUES ('Rick',1200);
INSERT INTO Employee_Test VALUES ('John',1100);
INSERT INTO Employee_Test VALUES ('Stephen',1300);
INSERT INTO Employee_Test VALUES ('Maria',1400);


It is very easy to find the highest salary as:-
--Highest Salary
select max(Emp_Sal) from Employee_Test




Now, if you are asked to find the 3rd highest salary, then the query is as:-
--3rd Highest Salary
select min(Emp_Sal) from Employee_Test where Emp_Sal in
(select distinct top 3 Emp_Sal from Employee_Test order by Emp_Sal desc)
The result is as :- 1200 


To find the nth highest salary, replace the top 3 with top n (n being an integer 1,2,3 etc.)
--nth Highest Salary
select min(Emp_Sal) from Employee_Test where Emp_Sal in
(select distinct top n Emp_Sal from Employee_Test order by Emp_Sal desc)