update table with data from other table if not null?

Question

Basically what I want to do is copy the value of a column from one table to another column in another table.

The query I am using is:

UPDATE t1 
SET prod         


        

Answer 1

You should just being doing the update across a join like this

UPDATE
t1 INNER JOIN t2 ON t1.variant_id = t2.variant_id
SET t1.product_code = t2.value
WHERE t2.key_id = 10
AND t2.value IS NOT NULL

There is no need to worry about nulls in that case as the inner join will only select rows where the variant_id exists in both tables.

Answer 2

Use the old value if the query returns null:

UPDATE t1 
SET product_code = 
ifnull(
    (SELECT `value` FROM t2 WHERE t2.variant_id = t1.variant_id AND key_id = 10), 
    product_code);

FYI, in mysql using the old value doesn't count as an "update" in any way (either the number of rows affected or the logged changes)