Oracle 从结果集中获取第一条记录

Question

我有一个类似的查询，下面给出了以下结果。对于一个给定的 pcid，可能有一个或多个图像。如果 pcid 返回了超过 1 张图像，我只想获取第一条记录。我知道 distinct 在这种情况下不起作用。任何想法将不胜感激。谢谢

select p.pcid,p.pc_name,p.pc_image_name
from pc p LEFT JOIN pc_goods pcm on p.pcid = pcm.pcid AND pcm.media_type = 'I' 
AND pcm.act_ind = 'Y' AND pcm.fea_ind = 'Y' INNER JOIN pc_cat pcat 
ON p.category_id = pcat.cat_id 
and p.fea_ind = 'Y' AND p.act_ind = 'Y'
order by pcid

Current results     
    pcid    pc_name   pc_image_name
    --------------------------------
    1120    testone   one.jpg
    1120    testwo    two.jpg
    1121    testthree three.png
    1125    testfour  four.png
    1128    test5     five.jpg
    1128    test6     six.jpg

想要的结果，

pcid    pc_name   pc_image_name
--------------------------------
1120    testone   one.jpg
1121    testthree three.png
1125    testfour  four.png
1128    test5     five.jpg

Answer 1

没有测试或解决为什么需要左连接... 如果 imageName 是唯一的，这样的事情可能会起作用。

SELECT p.pcid,p.pc_name,p.pc_image_name
FROM pc p 
LEFT JOIN pc_goods pcm 
  on p.pcid = pcm.pcid AND pcm.media_type = 'I' 
 AND pcm.act_ind = 'Y' AND pcm.fea_ind = 'Y' 
INNER JOIN (SELECT max(pc_image_name) maxImage, pcID from PC_GOODS group by PCID) PCM2
 on PCM2.maxImage=PCM.Pc_image_name 
and PCM2.PCID = PCM2.PCID
INNER JOIN pc_cat pcat 
  ON p.category_id = pcat.cat_id 
  and p.fea_ind = 'Y' AND p.act_ind = 'Y'
order by pcid

Answer 2

您可以为此使用 row_number window 函数：

row_number() over (partition by p.pcid order by null) as rn

...您按 pcid 划分的位置。由于您似乎并不关心选择了哪个图像，因此您可以按任何常量值排序（在这种情况下我使用了 null）。

select pcid,pc_name,pc_image_name
  from (
     select p.pcid,p.pc_name,p.pc_image_name,
            row_number() over (partition by p.pcid order by null) as rn
    from pc p LEFT JOIN pc_goods pcm on p.pcid = pcm.pcid AND pcm.media_type = 'I' 
    AND pcm.act_ind = 'Y' AND pcm.fea_ind = 'Y' INNER JOIN pc_cat pcat 
    ON p.category_id = pcat.cat_id 
    and p.fea_ind = 'Y' AND p.act_ind = 'Y')
 where rn = 1
 order by pcid

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