In SQL, adding a set of values corresponding on a join

Let's say that I have Table_A contain:

ID OldValue
3     16
1     5

and Table_B contains

ID Value OldValue
1    2     NULL
2    4     NULL 
3    8     NULL

and that I want to insert OldValues from Table_A into Table_B where the IDs 开发者_如何学运维equal resulting in :

updated Table_B

ID Value OldValue
1    2     5
2    4     NULL 
3    8     16

Is there a set based way of doing this without using a cursor? In real life this will be going on very large tables and cursors, as we all know are slow. Perhaps some sort of Merge?

I'm on SQL Server 2008

Update Table_B
Set OldValue = A.OldValue
From Table_A As A
    Join Table_B As B
        On B.ID = A.ID

You want an "update from"

UPDATE B
SET B.OldValue = A.OldValue
FROM Table_B B
INNER JOIN Table_A  ON B.ID = A.ID

Generally I write this as a select, then once i get the result I convert it over to the update.

Simple:

update x set OldValue = a.OldValue
from Table_B x
join Table_A a on x.ID = a.ID

Use a join:

UPDATE TABLE_B
SET OLDAVLUE=A.OLDVALUE
FROM TABLE_B B 
LEFT JOIN TABLE_A A ON B.ID=A.ID

Perhaps some sort of Merge?

Indeed. The following is Standard SQL that is supported on SQL Server 2008:

MERGE INTO Table_B
   USING Table_A
      ON Table_B.ID = Table_A.ID
WHEN MATCHED THEN
   UPDATE
      SET OldValue = Table_A.OldValue;