mysql - 子查询返回 Ifnull...NULL

Question

当我在 NULL 值上期望 0 时，为什么这个子查询返回 NULL 的任何想法:

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 总是返回 NULL

Answer 1

用 CASE 试试这个

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