Difference between ORIGINAL_LOGIN() and CURRENT_USER

ORIGINAL_LOGIN()  and CURRENT_USER are useful functions in  SQL Server , used to report on identities within SQL Server sessions.

The purpose of the ORIGINAl_LOGIN() is to return the value of the login which made the original connection to SQL Server.

CURRENT_USER - Returns the name of the current user. This function is equivalent to SELECT USER_NAME().  

The definitions  sound similar but there are some differences.    When using CURRENT_USER , the value will change according to different impersonations used.

In this example , we are using the EXECUTE AS functionality - which allows a login to execute code as another login - as long as the right permissions are in place. If you run this code , assuming you have a login called "jvtest" - , you'll see the 2nd instance current user , changes to the value of the EXECUTE AS , in other words , an impersonation is in progress.

You may find that in all the cases of select CURRENT_USER you may see "dbo". To read more about this situation read select current_user shows dbo

But the ORIGINAL_LOGIN() within the EXECUTE AS functionality - will return the login used to originally connect to SQL Server.

--1st current_user

select current_user
EXECUTE AS login = 'jvtest'

--2nd current user

select current_user
select original_login()

REVERT

--3rd current user
select current_user

The ORIGINAL_LOGIN() is useful when troubleshooting issues when impersonation is adopted , and is also a useful value to record in SQL Server security audits.