Question

In one statement I’m trying to group rows of one table by joining to another table. I want to only get grouped rows where their grouped result is not empty.

Ex. Items and Categories

SELECT Category.id
FROM Item, Category
WHERE Category.id = Item.categoryId
GROUP BY Category.id
HAVING COUNT(Item.id) > 0

The above query gives me the results that I want but this is slow, since it has to count all the rows grouped by Category.id.

What’s a more effecient way?

I was trying to do a Group By LIMIT to only retrieve one row per group. But my attempts failed horribly. Any idea how I can do this?

Thanks

Answer 1

Try this:

SELECT  item.categoryid
FROM    Item
JOIN    Category
ON      Category.id = Item.categoryId
GROUP BY
        item.categoryid
HAVING  COUNT(*) > 0

This is similar to your original query, but won’t do what you want.

If you want to select non-empty categories, do this:

SELECT  category.id
FROM    category
WHERE   id IN
        (
        SELECT  category_id
        FROM    item
        )

For this to work fast, create an index on item (category_id).