How can I add a sequence to the left of a materialized view in SQL?

Question

I have a materialized view as follows:

|product_type|
--------------
shoes
clothing
electronics
..100's more

I'd like to create the view instead in a way that each product type has a corresponding ID. So the view should look as follows:

| ID | product_type|
--------------------
1   | shoes
2   | clothing
3   | electronics
100 | ..100's more

SQL used to create the view was:

select distinct(product_type) from items

Answer 1

You could use row_number():

select row_number() over (order by product_type) as my_id

However, if you need a unique number per product_type -- and one that is stable over time -- I would suggest using an identity column in a "product types" table.

For your particular example, I would suggest:

select row_number() over (order by product_type), product_type
from items
group by product_type;