Question

当我期望NULL 0值NULL时，此子查询返回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的任何想法：

keyword_last_visit

NULL始终返回{{1}}

Answer 1

使用CASE

尝试此操作

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

这应该用在外部查询..

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

ifnull on subquery返回... NULL

2 个答案: