Try:
UPDATE table SET
column1 = 'value1'
WHERE column2 = 'value1' AND (column1 = '' OR column1 IS NULL);

Thanks so much LifeinAsia

That was the one, I had been playing around with it and never getting the proper command :(

Thanks again

-Gs

SET is an sql command that will assign a value to the variable. Thanks for sharing, it was a quick brush up for me.

why not

update table
set column1=column2
WHERE column1 = '' OR column1 IS NULL

why not ...

Because that would set the value for EVERY column1 (when blank or null), regardless of the value of column2. If the OP only wants to change the rows where column2 equals value1, but not where column2 equals some other value, code will do what is needed. For changing EVERY column1, your code is the correct method.

Reading the OP again, I see that I focused on the query, not the text explanation. Good catch!

To add to the mix . . . if you set up your database correctly, you will have an expected value for uninitialized field.s That is, if NULL does it for you, you need only test for null.

UPDATE table SET
column1 = 'value1'
WHERE column2 = 'value1' AND column1 IS NULL;

If there's a possibility you'd insert a blank string in there, then it will no longer be NULL, so go with the or . . .