Databases
StackOverflow2013
Postsdbo
POSQL greatest-n-per-group with relational table joins
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POSQL greatest-n-per-group with relational table joins
    primarykey
    data
    text
    <p>I have 3 tables. image, categories, image_category.</p> <pre><code>image: id | title | imageURL categories: cat_id | cat_name image_category: image_id | cat_id </code></pre> <p>My current query to select all images in order newest to oldest is:</p> <pre class="lang-sql prettyprint-override"><code>SELECT image.id as ID, image.title as title, categories.cat_name as CAT FROM image_category LEFT JOIN image ON image_category.image_id = image.id INNER JOIN categories ON image_category.cat_id = categories.cat_id ORDER BY ID DESC </code></pre> <p>I would like to show the newest 4 images per category. The largest image.id are the newest images. </p> <p>For example. If I had 3 categories and 40 images in each category. I want to show the newest 4 images from each category. I will later be trying to show the next 4 per category after that and then the next 4 per category until there are no images left.</p> <p><a href="https://stackoverflow.com/questions/1442527/how-to-select-the-newest-four-items-per-category">This</a> solution seems like what im looking for.</p> <pre class="lang-sql prettyprint-override"><code>SELECT i1.* FROM item i1 LEFT OUTER JOIN item i2 ON (i1.category_id = i2.category_id AND i1.item_id &lt; i2.item_id) GROUP BY i1.item_id HAVING COUNT(*) &lt; 4 ORDER BY category_id, date_listed; </code></pre> <p>but I have a relational table connecting my image_id and category_id. Cant figure out how to implement this with that extra table join.</p> <p>Would appreciate help from an SQL guru.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USCommunity
    1. USchris
    plurals
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POSQL greatest-n-per-group with relational table joins
      1. This table or related slice is empty.
      1. VTUpMod
    1. COPlease post table schemas and sample data
      singulars
      1. POSQL greatest-n-per-group with relational table joins
      1. USpeterm
    2. COcan an image have many categories? (since it looks like a many-to-many relation)
      singulars
      1. POSQL greatest-n-per-group with relational table joins
      1. USCrowbarKZ
    3. COgreatest n per group ?? group by what?? and what is the criteria to be greatest?
      singulars
      1. POSQL greatest-n-per-group with relational table joins
      1. USchetan
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload