SQL Server - Select rows of a "max date" from a table (using ROW_NUMBER() OVER)

I often come across different requirements where I need to get the "top row" where certain criteria exists. It often applies to VAT Rates, Exchange Rates, even historical data where a different record may apply depending on the scenario. For this example, let's use VAT rates.

Let's create sample data using the following:

create table #myvattable (vatname nvarchar(50), effectivedate datetime)
insert #myvattable values('Rate A', '01/01/2009')
insert #myvattable values('Rate B', '01/01/2009')
insert #myvattable values('Rate C', '01/01/2009')
insert #myvattable values('Rate A', '01/02/2009')
insert #myvattable values('Rate B', '01/02/2009')
insert #myvattable values('Rate B', '01/02/2008')


So, we have several instances of the same "Rate" in the table that have different effective dates. Now, let's take this requirement:

I want to retrieve all VAT rates applicable for a certain date. How do we do this?


Step 1 - use a row_number() clause on your dataset:

select * from
(
select row_number() over (partition by a.vatname order by a.vatname, a.effectivedate desc) as rownum,
a.vatname, a.effectivedate
from #myvattable a
)
tbl



This simply puts a row_number clause on each row partitioning by the vatname. This returns a result set similar to this:

rownum vatname effectivedate
------ ------------- -----------------------
1 Rate A 2009-01-02 00:00:00.000
2 Rate A 2009-01-01 00:00:00.000
1 Rate B 2009-01-02 00:00:00.000
2 Rate B 2009-01-01 00:00:00.000
3 Rate B 2008-01-02 00:00:00.000
1 Rate C 2009-01-01 00:00:00.000


Ok, so what does this do for us? It effectively ranks the rows by each vatname so that we can see which ones are most applicable. Basically we want all the 1's.

Step 2 - Filter the rownums.
So our query becomes this:

select * from
(
select row_number() over (partition by a.vatname order by a.vatname, a.effectivedate desc) as rownum,
a.vatname, a.effectivedate
from #myvattable a
)
tbl
where rownum = 1


Ok, pretty much there now. The only step really that is left is... what if someone wants historical data? This query is fixed to the latest most applicable vatrate.


Step 3 - Make the date flexible

select * from
(
select row_number() over (partition by a.vatname order by a.vatname, a.effectivedate desc) as rownum,
a.vatname, a.effectivedate
from #myvattable a
where effectivedate <= @DATEVARIABLE )
tbl
where rownum = 1


So, @DATEVARIABLE could be a parameter/variable... or, even better, a field from another joined table.

How to calculate the width/height of a string in a particular font

- Follow up from the listbox width issue -

This applies to mainly asp.net, but the technique for winforms isn't drastically different. What we need to use is a Graphics object to measure a string width. Here's how:


// Bitmap is solely required for creating the graphics object
Bitmap TextBitmap = new Bitmap(1, 1);
Graphics g = Graphics.FromImage(TextBitmap);

// Create the font relative to the text (let's say Arial 8)
Font myFont = new Font("Arial", 8);
SizeF fontSize = g.MeasureString("This is a test string", myFont);



Done. The values you need are fontSize.Height and fontSize.Width.

How to close a window using ASP.Net server side

After doing what needs to be done you can register some startup script to close your browser window like this:

string closeScript = "<script language="javascript">window.top.close();</script>";
if (!ClientScript.IsStartupScriptRegistered("clientScript"))
  ClientScript.RegisterStartupScript(Page.GetType(), "clientScript", closeScript);

Getting the dialog arguments passed into a web page modal dialog

This is especially useful if you use Microsoft CRM. Because it ties in very closely to how the entity forms and grid views communicate to custom web pages etc.

For example, in Microsoft CRM you can edit the ISVConfig to add a button to the toolbar for the entity grid. Set the WinMode=2 (2 basically means a modal dialog) to allow CRM to provide you with the selected entities. So how do we know what items were selected in the entity list? It passes us in a list of IDs as part of the window.dialogArguments.

So, how do we get access to this? Through javascript. Here's the steps:

1. Create the following javascript function (available from MSCRM help pages):

function loadSelectedListItems() {
  var sGUIDValues = "";
  var selectedValues;
  //Make sure window.dialogArguments is available.
  if (window.dialogArguments) {
    selectedValues = new Array(window.dialogArguments.length - 1);
  }
  else {
    return
  }
  selectedValues = window.dialogArguments;
  if (selectedValues != null) {
    for (i = 0; i < selectedValues.length; i++) {
      sGUIDValues += selectedValues[i] + ",";
    }
    document.getElementById('<%= winDialogArguments.ClientID %>').value = sGUIDValues;
  }
}


2. Use this function as part of the onload of the body:
<body onload="loadSelectedListItems()">

3. Create a new holder object (in my example an asp textbox) in a hidden division on your form:
<div style="visibility: hidden">
   <asp:textbox id="winDialogArguments" runat="server" text="">
</div>



Job done. Just access the Text of the asp textbox server side on the next post back (e.g. click of an OK button) to get the list of IDs.

Web Page Modal Dialog opens new window on postback

If you have a web page that's opening as a Modal Dialog (using window.showModalDialog or similar) and you are finding your button post backs are opening up a new browser window... here's the code that generally stops it happening. At the top of your head tag insert the following:

<base target="_self" />

In some instances this won't work. If not then pop this onload script in the body:

<body onLoad="window.name='myDialogWindow' ......>


and set the base tag to this instead:

<base target="myDialogWindow" />

How to close a window with an asp button (ASP.Net) using Javascript

A really handy way to do this:

<a href="#" onClick="javascript:window.close();">
   <asp:Button ID="btnCancel" Text="Cancel" runat="server" />
</a>