update e
set    col1 =  m.col1
from   excel_table e inner join master_table m
       on e.pk = m.pk
where  e.col1 is null

quote:

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OLE DB Provider for Jet
The Microsoft® OLE DB Provider for Jet provides an OLE DB interface to Microsoft Access databases, and allows Microsoft SQL Server™ 2000 distributed queries to query Access databases.

To create a linked server to access an Access database

Execute sp_addlinkedserver to create the linked server, specifying Microsoft.Jet.OLEDB.4.0 as provider_name, and the full path name of the Access .mdb database file as data_source. The .mdb database file must reside on the server. data_source is evaluated on the server, not the client, and the path must be valid on the server.
For example, to create a linked server named Nwind that operates against the Access database named Nwind.mdb in the C:\Mydata directory, execute:

sp_addlinkedserver 'Nwind', 'Access 97', 'Microsoft.Jet.OLEDB.4.0',
'c:\mydata\Nwind.mdb'

To access an unsecured Access database, SQL Server logins attempting to access an Access database should have a login mapping defined to the username Admin with no password.
This example enables access for the local user Joe to the linked server named Nwind.

sp_addlinkedsrvlogin 'Nwind', false, 'Joe', 'Admin', NULL

To access a secured Access database, configure the registry (using the Registry Editor) to use the correct Workgroup Information file used by Access. Use the Registry Editor to add the full path name of the Workgroup Information file used by Access to this registry entry:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Jet\4.0\Engines\SystemDB

After the registry entry is configured, use sp_addlinkedsrvlogin to create login mappings from local logins to Access logins:

sp_addlinkedsrvlogin 'Nwind', false, 'Joe',
'AccessUser', 'AccessPwd'

Access databases do not have catalog and schema names. Therefore, tables in an Access-based linked server can be referenced in distributed queries using a four-part name of the form linked_server...table_name.

This example retrieves all rows from the Employees table in the linked server named Nwind.

SELECT *
FROM Nwind...Employees

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