问题描述
我试图在每条记录的三个不同字段中选择最大日期 (MySQL)所以，在每一行中，我有 date1、date2 和 date3:date1 总是被填充，date2 和 date3 可以是 NULL 或空GREATEST 语句简单明了，但对 NULL 字段没有影响，因此效果不佳:
I'm trying to select the max date in three different fields in each record (MySQL) So, in each row, I have date1, date2 and date3: date1 is always filled, date2 and date3 can be NULL or empty The GREATEST statement is simple and concise but has no effects on NULL fields, so this doesn't work well:
SELECT id, GREATEST(date1, date2, date3) as datemax FROM mytable
我还尝试了更复杂的解决方案，例如:
I tried also more complex solutions like this:
SELECT
CASE
WHEN date1 >= date2 AND date1 >= date3 THEN date1
WHEN date2 >= date1 AND date2 >= date3 THEN date2
WHEN date3 >= date1 AND date3 >= date2 THEN date3
ELSE date1
END AS MostRecentDate
同样的问题:NULL 值是返回正确记录的一个大问题
Same problem here: NULL values are a GREAT problem in returning the right records
请问，你有解决办法吗?提前致谢....
Please, have you got a solution? Thanks in advance....
推荐答案
使用COALESCE
SELECT id,
GREATEST(date1,
COALESCE(date2, 0),
COALESCE(date3, 0)) as datemax
FROM mytable
更新:这个答案以前使用了 IFNULL ，它确实有效，但正如 Mike Chamberlain 在评论中指出的那样，COALESCE 实际上是首选方法.
Update: This answer previously used IFNULL which does work, but as Mike Chamberlain pointed out in the comments, COALESCE is actually the preferred method.
这篇关于MySQL:获取多列的 MAX 或 GREATEST，但包含 NULL 字段的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持编程学习网！