MySQL的左联接不返回空行

Question

I'm trying to get the list of categories with number of child records present in there. If the categories doesn't have records it should return NULL or 0 but my query returning categories with child records looks like its skipping the one without child records. ... will really appreciate the help.

here's my code:

SELECT 
t_gal.f_sub_category_id, 
t_sub_cat.f_sub_cat_name, 
t_gal.f_image_thumb, (
    SELECT COUNT(*)
    FROM t_gallery
    WHERE f_sub_category_id = t_gal.f_sub_category_id) 
    AS f_image_total
FROM    t_gallery t_gal
LEFT JOIN t_sub_category t_sub_cat ON t_sub_cat.r_id = t_gal.f_sub_category_id
GROUP BY t_sub_cat.r_id
ORDER BY t_gal.f_added_on DESC, t_gal.r_id DESC

Here's the two tables:

Answer 1

Problem appears to be your group by clause.

You are grouping by a field that is on the LEFT JOINed table, hence when it does the group by all the rows which do not have a matching row on that table would appear to be aggregated into a single row.

I think what you are trying to get is a list of gallery items, along with the category they are in (if found) and the count of other galleries in the same category. If so try the following (if not let me know!)

SELECT t_gal.f_sub_category_id, t_sub_cat.f_sub_cat_name, t_gal.f_image_thumb, Sub1.GalleryCount
FROM    t_gallery t_gal
LEFT JOIN t_sub_category t_sub_cat 
ON t_sub_cat.r_id = t_gal.f_sub_category_id
LEFT OUTER JOIN (SELECT f_sub_category_id, COUNT(*) AS GalleryCount FROM t_gallery GROUP BY f_sub_category_id) Sub1
ON Sub1.f_sub_category_id = t_gal.f_sub_category_id
ORDER BY t_gal.f_added_on DESC, t_gal.r_id DESC

Answer 2

It LOOKS like for every sub-category (of a previously selected category), you want to include ALL of that sub-category... And, of those sub-categories, you want a count of how many images for that category wheather or not there even IS an image in the gallery table.

What you may have been running into is the select statement for the FIELD used to count images... first, that could become a performance killer. Instead, you could just do a left-join directly to the gallery table and COUNT the distinct R_IDs from the gallery FOR the corresponding sub-category

SELECT 
      t_sub_cat.r_id,
      t_sub_cat.f_sub_cat_name, 
      MIN( COALESCE( t_gal.f_image_thumb, '' )) as JustOneThumbImg, 
      COUNT( DISTINCT( t_gal.r_id )) SubCategoryImageCount
   FROM 
      t_sub_category t_sub_cat 
         LEFT JOIN t_gallery t_gal
            ON t_sub_cat.r_id = t_gal.f_sub_category_id
   GROUP BY 
      t_sub_cat.r_id
   ORDER BY 
      t_sub_cat.f_added_on DESC

Since you are not grabbing all gallery images (since some may not exist FOR a given sub-category), ordering by the t_gal.r_id doesn't make sense

Also, the reason I'm not pre-grabbing aggregates in a sub-query to join against... I don't want to get everything from every category / sub-category without knowing which sub-categories are associated with the category you actually want.

Answer 3

the problem with your query is that you are using t_gallery as your main table and not t_sub_category while using left join.

you could try this: sqlfiddle

select 
t_gal.f_sub_category_id, 
t_sub_cat.f_sub_cat_name, 
 (
    SELECT COUNT(*)
    FROM t_gallery
    WHERE f_sub_category_id = t_gal.f_sub_category_id) 
    AS f_image_total
from t_sub_category as t_sub_cat
left join t_gallery t_gal on t_gal.f_sub_category_id = t_sub_cat.r_id
GROUP BY t_sub_cat.r_id
ORDER BY t_gal.r_id DESC;