UPDATE updated_profiles
SET last_updated = b.first_request
,category1 = CASE WHEN (b.category1 IS NULL) THEN a.category1 ELSE b.category1 END
,category1_perc = CASE WHEN (b.category1 IS NULL) THEN a.category1_perc ELSE b.category1_perc END
,category2 = CASE WHEN (b.category1 IS NULL) THEN a.category2 ELSE b.category2 END
,category2_perc = CASE WHEN (b.category1 IS NULL) THEN a.category2_perc ELSE b.category2_perc END
,category3 = CASE WHEN (b.category1 IS NULL) THEN a.category3 ELSE b.category3 END
,category3_perc = CASE WHEN (b.category1 IS NULL) THEN a.category3_perc ELSE b.category3_perc END
...

CASE WHEN (b.category1 IS NULL)
THEN
SET category1 = a.category1,
category1_perc = a.category1_perc
...
ELSE
SET category1 = b.category1,
category1_perc = b.category1_perc
END

UPDATE xxx
SET category1 = a.category1
...
WHERE b.category1 IS NULL
 
UPDATE xxx
SET category1 = b.category1
...
WHERE b.category1 IS NOT NULL