Updating a table in SQL Server with information from another table

I have a “claims” table. Each claim has a client attached to it which is represented by a few letters “ABED”. For some reason, the actual clientID is nowhere to be found in the claims table so I want to fix this. The “Clients” table DOES contain both the code AND clientID. How do I run a query where I can update existing claims with the clientID based on the code?

UPDATE claims 
SET clientID = (SELECT clientID FROM clients WHERE claimID = code) 

In the claims table, the code for the client is called “claimID”. In the clients table it’s called code.

Just to be clear, I did add the clientID column in the claims table, it is currently NULL and I’m looking to fill it with the clientID from the clients table.

Answer

Is this what you want?

update cla
set clientid = (select t.clientid from clients cli where cli.code = cla.claimid)
from claims cla

This searches for clients whose code matches the claimid of claims, and updates the corresponding clientid.