Joining with NULL values - mysql

Question

How deals with MySQL JOIN deal with NULL values. Being that most address do not have a second line address, there are many NULL values for address2. When I run the query to match up address based on address1, address2 and zip, the query will not return positive matched of when only address1, and zip match. I have ensured that both tables have a value of NULL for address2, when they are NULL. So basically to run the query I run two queries.
UPDATE target t
JOIN pl2.site2 s ON t.address1=s.address1 AND t.zip5=s.zip5
SET t.idsite=s.idsite
WHERE t.address2 IS NULL AND s.address2 IS NULL;
UPDATE target t
JOIN pl2.site2 s ON t.address1=s.address1 AND t.address2=s.address2 AND t.zip5=s.zip5
SET t.idsite=s.idsite;
Not really a hard work around. Just trying to get more into the science of how the SQL works. Maybe there was another solution, I don't want to set the columns=''(blank) where they are NULL either.

Answer 1

You have a couple of options to join the null fields. One is to use COALESCE which converts the null values to blanks for the comparison:
UPDATE target t
JOIN pl2.site2 s ON
t.address1=s.address1 AND
COALESCE(t.address2,'')=COALESCE(s.address2,'') AND
t.zip5=s.zip5
SET t.idsite=s.idsite;
Another option would be to use OR in the JOIN:
UPDATE target t
JOIN pl2.site2 s ON
t.address1=s.address1 AND
t.zip5=s.zip5 AND
((t.address2=s.address2) OR (t.address2 IS NULL AND s.addresss2 IS NULL))
SET t.idsite=s.idsite;