SQL - 使用Count（）Group By显示所有空行

Question

These are my tables:

Categories table
================
id (fk)
category_name


Items table
===========
id (pk)
item_name
category_id (pk)

One category has many Items

One item have one category

Let's say I have these data:

Categories
==========
id       category_name
-----------------------
1        Foods
2        Beverages
3        Computer
4        Cats


Items
=====
id    item_name   category_id(fk)
1     Rice        1
2     Chicken     1
3     Mouse       3
4     Keyboard    3

Query that I used to count items grouped by category name:

SELECT
    categories.id,
    categories.category_name,
    COUNT(items.item_name) AS items
FROM
    items
INNER JOIN categories ON items.category_id = categories.id
GROUP BY
    category_name

I've tried the above query to display the counting, but it doesn't show all rows from Categories table. Well, of course some item might not be in a category, but how do I show the empty Category as well?

Answer 1

could you try this?

SELECT
    categories.id,
    categories.category_name,
    COUNT(items.item_name) AS items
FROM
    categories LEFT JOIN items
        ON items.category_id = categories.id
GROUP BY
    categories.id, category_name

to print out ONLY empty categories

SELECT
    categories.id,
    categories.category_name
FROM
    categories LEFT JOIN items
        ON items.category_id = categories.id
WHERE items.category_id IS NULL;