How to count records using an l-tree - sql

Question

I have two tables, tickets and categories. The categories table has 3 columns of interest: id, name and path. The data looks like this:
id | Name | Path
------------------
1 | ABC | 1
2 | DEF | 1.2
3 | GHI | 1.2.3
4 | JKL | 4
5 | MNO | 4.5
6 | PQR | 4.5.6
9 | STU | 4.5.9
Note that the path column is an l-tree. What this is meant to represent is that the category with id=2 is a subcategory of id=1 and that id=3 is a subcategory of id=2.
In my tickets table, there's a column called category_id which refers to the id column in my categories table. Each ticket can have up to one category assigned to it (category_id may be null).
I'm trying to count all the tickets for each category.
Suppose my tickets table looks like this:
ticket_id | ticket_title | category_id
1 | A | 1
2 | B | 2
3 | C | 3
4 | D | 5
5 | F | 5
6 | G | 6
7 | H | 9
I would like to output:
category_id | count
1 | 3
2 | 2
3 | 1
4 | 4
5 | 4
6 | 1
9 | 1
I've found that I can get all of the tickets which belong to a given category with the following query: select * from tickets where category_id in (select id from categories where path ~ '*.1.*'); (although now that I'm writing this question I'm not convinced this is correct).
I've also attempted to perform the ticket-count-by-category problem and I came up with:
SELECT
categories.id as cid,
COUNT(*) as tickets_count
FROM tickets
LEFT JOIN categories ON tickets.category_id = categories.id
GROUP BY cid;
which outputs the following:
c_id | count
1 | 1
2 | 1
3 | 1
5 | 2
6 | 1
9 | 1
I'm not very good at SQL. Is it possible to achieve what I want?

Answer 1

Try this:
WITH tickets_per_path AS (
SELECT
c.path AS path,
count(*) AS count
FROM tickets t INNER JOIN categories c ON (t.category_id = c.id)
GROUP BY c.path)
SELECT
c.id,
sum(tickets_per_path.count) AS count
FROM categories c LEFT JOIN tickets_per_path ON (c.path #> tickets_per_path.path)
GROUP BY c.id
ORDER BY c.id;
Which yields the following result:
id| count
1 | 3
2 | 2
3 | 1
4 | 4
5 | 4
6 | 1
9 | 1
It roughly works like this:
the WITH clause computes the number of tickets per path (without
including the count of tickets of descendent paths).
the second select clause joins the categories table with the precomputed tickets_per_path view, but instead of an equi-join on path, it
joins by testing whether a record in the left table (categories) is
an ancestor of the right side table (using #> operator). Then it
groups by category id and sums up the ticket counts by category
including the descendant counts.

Answer 2

You are close, but you need a more general join:
SELECT c.id as cid, COUNT(*) as tickets_count
FROM categories c LEFT JOIN
tickets t
ON t.category_id || '.' LIKE c.id || '.%'
GROUP BY c.id;
The '.' in the comparison is just so 1.100 doesn't match 1.1.