SQL Server Execute Impersonation

execute as login provides impersonation to the entire server, since logins are on a server level. Since users are defined per database, execute as user impersonation applies only to a specific database, which is why you see the error when you cross databases.

The EXECUTE AS can be added to stored procs, functions, triggers, etc.

Example to Execute As:

CREATE PROCEDURE dbo.MyProcedure
      WITH EXECUTE AS OWNER

In this case you are impersonating the owner of the module being called.

You can also impersonate SELF, OR the USER creating or altering the module OR... impersonate CALLER , which will enable to module to take on the permissions of the current user, OR... impersonate OWNER, which will take on the permission of the owner of the procedure being called OR... impersonate 'user_name', which will impersonate a specific user OR... impersonate 'login_name' with will impersonate a specific login.

Setting permission on objects like stored procedures can be accomplished with

GRANT EXECUTE ON <schema>.<procedurename> to <username>; 

However, you may also want to grant security rights at both the login and user level.

You will want to determine and grant ONLY the necessary rights for the objects that require access (such as execution). Consider use of the "EXECUTE AS" capability which enables impersonation of another user to validate permissions that are required to execute the code WITHOUT having to grant all of the necessary rights to all of the underlying objects (e.g. tables).

MOST of the time, you will only need to grant EXECUTE rights to stored procs and then rights are granted to all objects referenced within the stored proc.

In this way, you do not need to give implicit rights (example: to update data or call additional procs). Ownership chaining handles this for you. This is especially helpful for dynamic sql or if you need to create elevated security tasks such as CREATE TABLE. EXECUTE AS is a handy tool to consider for these.

This example may help clarify all of this:

--Create a user called NoPrivUser with public access to a database (e.g. dbadb)

USE [master]
GO
CREATE LOGIN [NoPrivUser] WITH PASSWORD=N'ABC5%', DEFAULT_DATABASE=[dbadb], CHECK_EXPIRATION=ON, CHECK_POLICY=ON
GO
USE [DBAdb]
GO
CREATE USER [NoPrivUser] FOR LOGIN [NoPrivUser]
GO

NOTE: CREATOR OR OWNER OF THIS PROCEDURE WILL REQUIRE CREATE TABLE RIGHTS within the target database.

use DBAdb
go
CREATE PROCEDURE dbo.MyProcedure 
WITH EXECUTE AS OWNER
AS
IF NOT EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].MyTable') AND type in (N'U')) 
CREATE TABLE MyTable (PKid int, column1 char(10)) 
INSERT INTO MyTable
VALUES (1,'ABCDEF')

GO

GRANT EXEC ON dbo.MyProcedure TO NoPrivUser;
GO

-- Now log into your database server as NoPrivUser and run the following.

use dbadb
go

EXEC dbo.MyProcedure

--(1 row(s) affected)

Now try to select from the new table while logged on as NoPrivuser.

You will get the following:

select * from MyTable
go

Msg 229, Level 14, State 5, Line 1 The SELECT permission was denied on the object 'MyTable', database 'DBAdb', schema 'dbo'.

That is expected since you only ran the procedure under the security context of Owner while logged on as NoPrivUser.
NoPrivUser as no rights to actually read the table, Just to execute the procedure which creates and inserts the rows.

With the EXECUTE AS clause the stored procedure is run under the context of the object owner. This code successfully creates dbo.MyTable and rows are inserted successfully.

In this example, the user "NoPrivUser" has absolutey no granted rights to modify the table, or read or modify any of the data in this table.
It only takes on the rights needed to complete this specific task coded WITHIN the context of this procedure.

This method of creating stored procedures that can perform tasks that require elevated security rights without permanently assigning those rights come be very useful.

Login scope is at the server level while user scope is at the current database level

http://msdn.microsoft.com/en-us/library/ms181362.aspx