And next time use a BEGIN TRAN so you can roll back if needed.

FYI, when making changes to data like this, I usually take a few precautions.

1. Back up the table first by creating a copy with a different name, such as:

select * into Table_20090205

from Table

2. Run a select to see how many records will get updated and Review the data to make sure it looks like the right records:

select * FROM Table_20090205

WHERE ColumnA Is Null

3. I would run the update in a transaction so you can undo it right then if you make a mistake:

begin tran

UPDATE T SET ColumnA = 'NewValue'

FROM Table T

WHERE ColumnA Is Null

rollback

commit

If you get the correct count, then: COMMIT

If you think it's wrong and need to undo, then : ROLLBACK

If you accidentally run the script all at once, having the rollback in there will undo it

4. When you're all finished, save the whole script somewhere in case you need to look back sometime.

Record the number of rcds changed in the script