SQL - Concatenating values when the number of items is not known - sql

Question

I am currently refreshing my knowledge in SQL and encountered some difficulties with the following query.
The requirements were:
For each maker, list in the alphabetical order with "/" as delimiter all the types of products he produces.
Deduce: maker, product types' list
The following solution actually works but I don't exactly understand how..
;with
t1 as
(select maker, type, DENSE_RANK() over(partition by maker order by type) rn
from product
),
tr(maker, type,lev) as
(select distinct t1.maker, cast(t1.type as nvarchar) , 2 from t1 where t1.rn = 1
union all
select t1.maker, cast(tr.type +'/'+t1.type as nvarchar), lev + 1
from t1 join tr on (t1.maker = tr.maker and t1.rn = tr.lev
)
)
select maker, max(type) names from tr group by maker
These output:
1 | A | Laptop/PC/Printer
2 | B | Laptop/PC
3 | C | Laptop
4 | D | Printer
5 | E | PC/Printer
*second column is the maker and third is the dynamically concatenated list of types.
Now, I'm a bit confused on how exactly does the lev grows dynamically.. Is there some kind of loop I'm missing here?
Why does it start with 2?
Why it doesn't work without the "cast"?
I would be very grateful if someone could explain the logic behind this query.
Thanks a lot!

Answer 1

What you're looking at is a recursive CTE,a CTE that calls itself. It's what you think is the "looping" effect. Recursion put a kink in my brain when I first looked at it. It helps to look at some examples and try creating a few simple ones of your own. I'm still not the best at them, but I'm getting better. I added some comments to your code. Hope this helps.
;WITH t1
AS (
SELECT maker,
type,
--Partition says each maker is a group so restart at 1
--Order by type is alphabetic
--DENSE_RANK() means if there are two the of the same item, they get the same number
DENSE_RANK() OVER (PARTITION BY maker ORDER BY type) rn
FROM product
),
--This is a recursive CTE meaning a CTE that calls itself(what is doing the "looping"
tr (maker,type,lev)
AS (
--Grab only the distinct items from your ranked table
SELECT DISTINCT t1.maker,
cast(t1.type AS NVARCHAR),
2 --This is the start of your recursive loop
FROM t1
WHERE t1.rn = 1
UNION ALL
--Recursively loop through you data, adding each string at the end with the '/'
SELECT t1.maker,
cast(tr.type + '/' + t1.type AS NVARCHAR),
--Plus one to grab the next value
lev + 1
FROM t1
INNER JOIN tr ON (
--Only match the same makers
t1.maker = tr.maker
--Match to the next value
AND t1.rn = tr.lev
)
)
--I think you know what this does
SELECT maker,
max(type) names
FROM tr
GROUP BY maker