MySQL DISTINCT放在一栏和左联接

Question

I have a SQL query with a left join:

SELECT i.id
     , i.user_id
     , i.date
     , h.cat_id
  FROM io__image i
  LEFT 
  JOIN io__image_cat_hext h
    ON h.image_id = i.id
 WHERE i.home_granted >= 1 
   AND user_id = 40
 ORDER 
    BY i.date DESC

Result looks like:

id   user_id          date       cat_id     
530     40  2018-05-12 20:45:54     42
528     40  2018-05-11 22:59:17     42
518     40  2018-05-11 17:22:30     42
508     40  2018-05-09 13:45:40     37
504     40  2018-05-06 22:40:31     37
492     40  2018-05-02 21:21:20     37
490     40  2018-05-02 16:16:09     36
481     40  2018-05-02 15:08:35     36

Would it be possible to get DISTINCT cat_id? I tried with Group by:

SELECT i.id
     , i.user_id
     , i.date
     , h.cat_id
  FROM io__image i 
 GROUP 
    BY h.cat_id
  LEFT 
  JOIN io__image_cat_hext h
    ON h.image_id = i.id
WHERE i.home_granted >= 1 
   AND user_id = 40
 ORDER 
    BY i.date DESC

But I get this error:

Syntax near 'LEFT JOIN io__image_cat_hext ON io__image_cat_hext.image_id = io__image.id WHER' on line 4

Answer 1

It's hard to understand what you exactly mean by DISTINCT cat_id in this context. Maybe you are searching for something like this?

SELECT io__image.id,
       io__image.user_id,
       io__image.date,
       io__image_cat_hext.cat_id
       FROM (SELECT max(io__image_cat_hext.image_id) image_id,
                    io__image_cat_hext.cat_id
                    FROM io__image_cat_hext
                    GROUP BY io__image_cat_hext.cat_id) io__image_cat_hext
            INNER JOIN io__image
                       ON io__image.id = io__image_cat_hext.image_id
       ORDER BY io__image.date DESC;

It'll give you only the image with the highest id for each category. If the ids are auto incremented, that's the image inserted most recently.

Answer 2

If you want only the cat_id values, then you can do:

select h.cat_id
from io__image i join
     io__image_cat_hext h
     on h.image_id = i.id
where i.home_granted >= 1 and i.user_id = 40
group by h.cat_id
order by max(i.date) desc;

Without showing the results you want, this seems like the best fit to your question.