MySQL：获取上次日期的记录

Question

I want to get all records which are not "older" than 20 days. If there are no records within 20 days, I want all records from the most recent day. I'm doing this:

SELECT COUNT(DISTINCT t.id) FROM t
WHERE 
(DATEDIFF(NOW(), t.created) <= 20 
OR 
(date(t.created) >= (SELECT max(date(created)) FROM t)));

This works so far, but it is awful slow. created is a datetime, might be due tue the conversion to a date... Any ideas how to speed this up?

Answer 1

SELECT COUNT(*) FROM (
 SELECT * FROM t WHERE datediff(now(),created) between 0 and 20
 UNION 
 SELECT * FROM (SELECT * FROM t WHERE created<now() LIMIT 1) last1
) last20d

I used the between clause just in case there might be dates in the future in the table. These will be excluded. Also you can simplify the select, if you just need the count() to

SELECT COUNT(*) FROM (
 SELECT id FROM t WHERE datediff(now(),created) between 0 and 20
 UNION 
 SELECT id FROM (SELECT id FROM t WHERE created<now() LIMIT 1) last1
) last20d

otherwise, in the first select version you can leave out the outer select if you want all the data of the chosen records. The UNION will make sure that duplicates will be excluded (in other cases I always use UNION ALL since it is faster).