Merge data from one table to another using UPDATE

Why the subquery? And use the preferred join syntax of table JOIN table ON

update  s
set     BarcodeRef = a.Barcode ,
        CustName = a.CustomerName ,
        CustCode = a.CustomerCode ,
        DocDate = a.[Date] ,
        Name = a.Name ,
        VehicleReg = a.VehicleRegistration ,
        Blanket = a.Blanket ,
        CustPONo = a.CustomerPO ,
        SOType = a.[Type]
from    tableDERP s
join    BOOKINGID a  on a.BookingID = s.BarcodeRef

An alternative to @kev riley's approach would be to do a MERGE statement, particularly if you suspect there's data missing. This only applies to SQL Server 2008 or later. See http://technet.microsoft.com/en-us/library/bb510625.aspx for more info.

Here's my preferred syntax i.e. using MERGE:

MERGE INTO tableDERP
   USING BOOKINGID AS a
      ON a.BookingID = tableDERP.BarcodeRef
WHEN MATCHED THEN
UPDATE
   SET BarcodeRef = a.Barcode
      ,CustName = a.CustomerName
      ,CustCode = a.CustomerCode
      ,DocDate = a.[Date]
      ,Name = a.Name
      ,VehicleReg = a.VehicleRegistration
      ,Blanket = a.Blanket
      ,CustPONo =  a.CustomerPO
      ,SOType = a.[Type];