具有order by和limit的mysql并集

a有这样的sql：

(select `ads`.* 
 from `ads`  
 where `status` = ? 
 and `lat` between ? and ? 
 and `lng` between ? and ? 
 and NOW() between ads.featured_start and ads.featured_end 
 order by `ads`.`id` desc) 
 union 
 (select `ads`.* 
  from `ads` 
  where `status` = ? 
  and `lat` between ? and ? 
  and `lng` between ? and ? 
  and NOW() not between ads.featured_start and ads.featured_end 
  order by `ads`.`id` desc) 
  limit 8 offset 0

但是，得到id=1,2,3,4,5的结果。。。这是必要的5,4,3,2,1 为什么？请帮帮我试试以下方法：

(select `ads`.* 
 from `ads`  
 where `status` = ? 
 and `lat` between ? and ? 
 and `lng` between ? and ? 
 and NOW() between ads.featured_start and ads.featured_end
 union 
 select `ads`.* 
  from `ads` 
  where `status` = ? 
  and `lat` between ? and ? 
  and `lng` between ? and ? 
  and NOW() not between ads.featured_start and ads.featured_end)
  ORDER BY id DESC
  limit 8 offset 0

代码模式是

( SELECT ... ORDER BY ... LIMIT .. )
UNION ALL? DISTINCT? -- Which do you need?
( SELECT ... ORDER BY ... LIMIT .. )
ORDER BY ... LIMIT .. OFFSET ..;

是的，这是……订购的3份

在外部，您有限制n偏移量m，但在内部使用限制m+n

使用3个副本的原因是为了最小化tmp表的大小-至少有3个tmp表