In this
article I am going to explain BCS in SharePoint 2010. How we can make an
external content type by using BCS. We will learn all of this step by step.
What is BCS ?
Microsoft
Business Connectivity Services (BCS), formerly named the Business Data Catalog,
enables users to read and write data from external systems—through Web
services, databases, and Microsoft .NET Framework assemblies—from within
Microsoft SharePoint 2010 and Microsoft Office 2010 applications
Business
Connectivity Services are a set of services and features that provide a way to
connect SharePoint solutions to sources of external data and to define external
content types that are based on that external data. This article contains a
step by step guide to design an External Content Type from SQL Server.
My SQL Server Data Base:

Image 1.
My Data Base Table in Design mode:

Image 2.
Now
next task is to create an Creating an external content type
Open
SharePoint Designer -> Open your site like below:

Image 3.
Click Open.

Image 4.
Select External Content types -> Click on External
Content Type option.

Image 5.
Now Click (click here to discover external data source
and define operation.)

Image 6.
Now Click on Add Connection -> Select SQL Server
-> OK.

Image 7.
Enter your Data Base Server Name, Data Base name,
Name(optional). And Click OK.

Image 8.
Here Expand your Data Table ->Right Click ->
Select Create All Operations

Image 9.
Click Next .

Image 10.
Set your table column as identifier-> Click Finish.

Image 11.
Now save this from Left top Save button.

Image 12.
Now open your site -> Select Site Actions -> More
options.

Image 13.
Select List -> External List -> Give name and
click on Create.

Image 14.
Now click on search and external content type -> a
pop up window will open -> search and select your External content type->
Ok.

Image 15.
Now Click Create.

Image 16.
Your SQL Server Data.
Now Select Item -> Add

Image 17.
Check your SQL Server Data Table also.
If there is an error message while displaying Data then
Go to Central Admin .

Image 18.
Select Manage Service Applications.

Image 19.
Select Business Data Connectivity Service.

Image 20.
Make sure Set Object Permission and Set Metadata Store
Permission should be like above.