select ... where (a=1 or b=min(b)) GROUP BY xy

Question

我有一张表 gall_photos，其中包含许多画廊的照片

• id (int)
• gall_id (int)
• 文件名（varchar）
• 位置（整数）
• 预览（tinyint 1）
• date_add（日期时间）

现在我想获取照片列表（用于摘要预览），每个 gall_id 一张，但如果有预览 1，那么这个，否则是位置​​编号最低的照片。最后按date_add排序

select * 
from gall_photos 
where (preview = 1 or position = min(position)) 
group by gall_id 
order by date_add

此查询返回错误（如果使用“或 position = min(position)”）

请问有什么办法吗？

编辑（12.1. 18:50UTC）：可以在一个画廊（gall_id）中查看更多照片的预览，然后我需要其中一张，再次是最低位置

Answer 1

你可以这样做

select * 
from gall_photos x 
where (preview = 1 or (position = 
                             (SELECT min(x2.position)
                              FROM gall_photos x2 
                              WHERE x.gall_id = x2.gall_id
                              Group By x2.gall_id)
                      and not exists 
                              (select 1 
                               from gall_photos x2 
                               where x2.gall_id=x.gall_id 
                               and x2.preview=1)
                        )
       );

SQL FIDDLE DEMO

Answer 2

rs 的解决方案非常接近，但是如果您有一条 preview=1 的记录，并且它没有 gall_id 的最低位置，他会给您 preview=1 和最低位置。所以我认为你想要的是：

select * 
from gall_photos g1
where preview = 1
  or not exists (select * from gall_photos g2 where g2.gall_id=g1.gall_id and g2.preview=1)
  and position = (select min(g3.position) from gall_photos g3
    where g3.gall_id=g1.gall_id)
order by date_add

Answer 3

这样的事情怎么样...

SELECT a.gall_id
     , COALESCE(c.position,a.position) position
  FROM gall_photos a
  JOIN 
     ( SELECT gall_id,MIN(position) min_position FROM gall_photos GROUP BY gall_id) b 
    ON b.gall_id = a.gall_id 
   AND b.min_position = a.position
  LEFT
  JOIN gall_photos c
    ON c.gall_id = a.gall_id
   AND c.preview = 1;

Answer 4

WHERE CLAUSE 不允许与GROUP BY 一起使用

使用HAVING：

select * 
from gall_photos 
group by gall_id 
having (preview = 1 or position = min(position)) 
order by date_add

Answer 5

select id, gall_id, filename, position, preview, date_add
from gall_photos 
where position in (select min(position)from gall_photos) 
or
exists (select preview from gall_photos where preview = 1)
group by gall_id
order by date_add;