mysql联合与order by和limit

Question

a have such 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

but, get results with id = 1,2,3,4,5... (it is necessary 5,4,3,2,1) why? help me please)

Answer 1

Try something like:

(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

The code pattern is

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

Yes that is 3 copies of ORDER BY ... .

Outside, you have LIMIT n OFFSET m, but inside use LIMIT m+n.

The reason for the 3 copies is to minimize the tmp table sizes -- there will be at least 3 tmp tables.