Need some SQL syntax help :-)
Both databases are on the same server
db1 = DHE
db2 = DHE_Import
UPDATE DHE.dbo.tblAccounts
INNER JOIN DHE_Import.dbo
You could call it just style, but I prefer aliasing to improve readability.
UPDATE A
SET ControllingSalesRep = RA.SalesRepCode
from DHE.dbo.tblAccounts A
INNER JOIN DHE_Import.dbo.tblSalesRepsAccountsLink RA
ON A.AccountCode = RA.AccountCode
For MySQL
UPDATE DHE.dbo.tblAccounts A
INNER JOIN DHE_Import.dbo.tblSalesRepsAccountsLink RA
ON A.AccountCode = RA.AccountCode
SET A.ControllingSalesRep = RA.SalesRepCode