Ifnull on subquery returns…NULL

Question

Any ideas why this subquery returns NULL when I'm expecting a 0 on NULL values:

SELECT
  (SELECT ifnull(visit.timestamp, 0) AS ifnull_1
   FROM keyword
   WHERE visit.keyword_id = keyword.id
   ORDER BY visit.timestamp DESC LIMIT 1) AS keyword_last_visit
FROM keyword
LEFT OUTER JOIN visit ON visit.keyword_id = keyword.id

keyword_last_visit returns always NULL

Answer 1

Try this with CASE

SELECT 
(SELECT (CASE WHEN visit.timestamp IS NULL THEN 0 ELSE visit.timestamp END) AS ifnull_1
   FROM keyword
   WHERE visit.keyword_id = keyword.id
   ORDER BY visit.timestamp DESC LIMIT 1) AS keyword_last_visit
FROM keyword
LEFT OUTER JOIN visit ON visit.keyword_id = keyword.id

Answer 2

This should be used on the outer query..

SELECT
  ifnull((SELECT ifnull(visit.timestamp, 0) AS ifnull_1
   FROM keyword
   WHERE visit.keyword_id = keyword.id
   ORDER BY visit.timestamp DESC LIMIT 1), 0) AS keyword_last_visit
FROM keyword
LEFT OUTER JOIN visit ON visit.keyword_id = keyword.id