Question

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）

Answer 1

尝试类似：

(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

Answer 2

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ORDER BY ...的3份副本。

在外面，你有LIMIT n OFFSET m，但内部使用LIMIT m + n。

3份副本的原因是最小化tmp表大小 - 至少有3个tmp表。

mysql联合与order by和limit

2 个答案: