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

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)