SQL GROUP BY Example

The GROUP BY clause causes data in a table (or any data source) to be divided into groups based on the expressions (or conditions) given in the GROUP BY clause. Seems confusing? Probably.. Let's see what it means,

Say you have Column1, Column 2..etc on your table.

Now you say GROUP BY Column1 , what this does is it divides your table's data based on Column1. So all data which has  Column1='Apple' will be in one group, then all data which has  Column1='Orange' will be in another group.

Again if you had GROUP BY Column1, Column2 then your data will be divided into groups of Column1 + Column2 combinations . If Column2 had values of Green and Red in different rows then one group would be based on Apple+Red another group Apple+Green. So you get the point.

So in a nutshell,

GROUP BY creates groups for output rows, according to unique combination of values specified in the GROUP BY clause

But why would you need to divide them into groups? So we can operate on those groups.

I'll use AdventureWorks database as an example.

Download it here,
https://msftdbprodsamples.codeplex.com/releases/view/93587

SELECT SalesPersonID, YEAR(OrderDate) AS OrderYear,

COUNT(CustomerID) AS All_Customers

FROM Sales.SalesOrderHeader

GROUP BY SalesPersonID, YEAR(OrderDate);

The above query will return all unique combinations of SalesPersonID  + YEAR(OrderDate) and display all customers each of those combinations.

In other words this means you are seeing all customers for a particular Sales Person in a given year.

Point to note:

SELECT DISTINCT SalesPersonID ,YEAR(OrderDate) AS OrderYear

FROM Sales.SalesOrderHeader

and

SELECT  SalesPersonID ,YEAR(OrderDate) AS OrderYear

FROM Sales.SalesOrderHeader

GROUP BY SalesPersonID, YEAR(OrderDate);

Will give you the same results.

Read More

ASP.Net Webforms Inline tags and code blocks

There are many elements inside a webforms (.aspx) page. To somebody beginning asp.net webforms these may seem quite similar and be confusing at first. (Well it was for me when I started learning asp.net webforms)
So hopefully this might help another soul.

Elements of a asp.net page (Inline tags and code blocks) can be listed as follows,

1. Directives

These come in many flavors like Page directives,
 ex : <%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs"  Trace="true" %>

or  Import directives,

ex : <%@ namespace="System.Drawing" %>

There are many more directives other than these two.

2. ASP.Net expressions

This is expression builder syntax, typically used to get values from web.config

ex: <%$ ConnectionStrings:AdventureWorks2008 %>  

3. User Controls

These are references to user created controls which are similar to web controls that enhances reusability 

ex: <PH:MainMenu Title="Menu Control" id="mainmenu" runat="server" />

4. Render blocks

Used for outputting elements like variables

ex:  If you have a variable named age then,

<%= age %>

will output the value of age

5. Code blocks

These are similar to javascript tags just that these run on the server,

<script runat="Server" language="c#" > ....some server side code here .... </script>  

6. Server Controls

These are the controls we drag and drop from the designer (Labels, Buttons etc)

ex: <asp:Label id="myLabel" runat="Server" />

7. Data binding controls

Used for biniding expressions such as Eval or Bind, these are typically used with data controls like Gridview
ex : <%# Eval("DBFieldName") %>

   

Read More

DataContract vs Serializable WCF

When applied to a class Serializable attribute serializes everything inside of it including private fields thus the developer has no control as what to expose.

When DataContract attribute is applied to a class it will serialize only the elements decorated with DataMember attribute thus the developer will have more control.

Read More

ServiceContract Name property WCF

This property can give your contract a name which is different from the actual interface name. So this way if you decide to change the interface name of a already running WCF service your client wont break.

Ex:

 [ServiceContract(Name="IGoodbyeService")]

    public interface IHelloService

    {

        [OperationContract]

        string GetMessage(string name);

    }

Now the contract name would be IGoodbyeService rather than  IHelloService.

To verify in the WSDL generated check the porttype property it should be IGoodbyeService

Note: Same can be done for OperationContract as well 

Read More

Basic Elements of WCF Appconfig

All config items should be inside ,

<system.serviceModel>

This is the WCF namespace provided by Microsoft.

Inside the <system.serviceModel> element you have the <services> element. This is where you configure all your services.
For each service you have a <service> element inside <services> element.
A particular service can be exposed using more than one endpoint. (Ex: http , net.tcp etc)
So for each endpoint in a service you configure a <endpoint> element.
An endpoint element will have three essential things,

1. address
2. binding
3. contract  

Here is a  sample config (not complete) to get an idea.

The service name is HelloService (class and namespace both are HelloService) and has two endpoints (http and net.tcp) for the service.

<?xml version="1.0" encoding="utf-8" ?>


<configuration>

  <system.serviceModel>

    <services>

      <service name="HelloService.HelloService">

        <endpoint address="HelloService" binding="basicHttpBinding" contract="HelloService.IHelloService">

        </endpoint>

        <endpoint address="HelloService" binding="netTcpBinding" contract="HelloService.IHelloService">

        </endpoint>

      </service>

    </services>

  </system.serviceModel>

</configuration>

Read More

system.web vs system.webserver

I have not worked in IIS6 and mostly I have worked on IIS 7+ and this came up when I was following a very old article (2003) ASP.Net article.

Anyway system.web is IIS6 and classic mode.
system.webserver is IIS7+  and integrated mode.

One scenario you might come across is when adding httpmodules to your ASP.Net project. If you want backward compatibility then you should add the module section to both system.web section (which is there by default for all ASP.Net projects ) and to system.webserver.

<system.web>
    <httpModules>
    </httpModules>
</system.web>

And

<system.webServer>
    <modules>
    </modules>
</system.webServer>

Read More

SQL JOIN Example

Joins can be quite perplexing initially hopefully this simple example will help. 

Joins are used to combine data from multiple tables (usually two tables)

There are three types of joins,

1. INNER

2. OUTER - FULL, LEFT, RIGHT

3. CROSS

For all examples I'll be using the below given tables and use the Name field as the condition.

                                                                         Table1

Id1	Name1
1	Padmika
2	Dissanayake
3	Sukitha
4	Pahan

                                                                        Table2

Id2	Name2
1	Fernando
2	Malaka
3	Padmika
4	Sukitha

1. INNER JOIN

Returns only the set of records that match the Name field in both Table 1 and Table2

SELECT * FROM Table1

INNER JOIN Table2

ON Table1.Name1 = Table2.Name2

Result,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
3	Sukitha	4	Sukitha

* This gets executed in the following manner, first get the first record from Table 1 get its name field compare it to Table 2 first record name field if it’s a match return it, now compare with Table2 second record name field…  etc.

You will see that only the records that have a common name is returned

This is similar to the intersection of two sets (Table1 and Table2) based on Name.  

2. OUTER JOIN

2.1 FULL OUTER JOIN

Returns all records in Table 1 and Table 2, with matching records from both sides where available. If there is no match, the missing side will contain null.

SELECT * FROM Table1

FULL OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Result,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
2	Dissanayake	null	Null
3	Sukitha	4	Sukitha
4	Pahan	null	Null
null	Null	1	Fernando
null	Null	2	Malaka

This is similar to the union of two sets (Table1 and Table2) based on Name.

2.2 LEFT OUTER JOIN

 Returns the complete set of records from Table 1 (Left table), with the matching records for Name field (where available) in Table 2. If there is no match, the right side will contain null.

SELECT * FROM Table1

LEFT OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Results,

Id1	Name1	Id2	Name2
1	Padmika	3	Padmika
2	Dissanayake	null	null
3	Sukitha	4	Sukitha
4	Pahan	null	null

Special case of the above result would be to select records that do not have any matching records from the right table (Table2)

To do this just add a ‘WHERE Table2.Id2 IS NULL’ clause to the above query.

Ex: You have a Customer table and an Order table (Has a CustomerID column) and want to find out which customers have not placed any orders  

2.3 RIGHT OUTER JOIN

Similar to the above but all the right table (Table2) records will return rather than the left table ones.

SELECT * FROM Table1

RIGHT OUTER JOIN Table2

ON Table1.Name1 = Table2.Name2

Results,

Id1	Name1	Id2	Name2
null	null	1	Fernando
null	null	2	Malaka
1	Padmika	3	Padmika
3	Sukitha	4	Sukitha

Note : When inspected carefully you will see that in this example,

OUTER JOIN result = LEFT JOIN result + RIGHT JOIN result – INNER JOIN result

Read More