Completing a table with rows not existing - sql

Question

I have a table like this:
buyer product quantity
tom skirt 2
anna skirt 3
tom jeans 5
The distinct(product) = skirt and jeans
I want a table that inserts another column with quantity = 0 when the <buyer, product> tuple does not exist for all possible products.
So the result would be:
buyer product quantity
tom skirt 2
anna skirt 3
tom jeans 5
anna jeans 0
It does not look very complicated but I don't know how to do it.
UPDATE
I have found one extra complication.
My product is actually defined by two fields: class and product. Product can be null and I need not to lose the information quantity when the product field is null (now it is happening with the cross join).
So if I have this:
buyer class product quantity
tom clothes skirt 2
anna clothes skirt 3
tom clothes jeans 5
jim shoes NULL 7
I would need:
buyer class product quantity
tom clothes skirt 2
anna clothes skirt 3
tom clothes jeans 5
anna clothes jeans 0
jim shoes NULL 7
jim clothes skirt 0
jim clothes jeans 0
tom shoes NULL 0
anna shoes NULL 0
Thank you all, I know I am complicating things!

Answer 1

You can use a cross join to generate all possible combinations of buyers and products. Then use a left join (or not exists) to filter out the ones already in the table:
insert into table(buyer, product, quantity)
select b.buyer, p.product, 0
from (select distinct buyer from table) b cross join
(select distinct product p from table) p left join
table t
on t.buyer = b.buyer and t.product = p.product
where t.buyer is null;
EDIT:
If you want a query that returns all the rows, then you would use something quite similar:
select b.buyer, p.product, coalesce(t.qty, 0) as qty
from (select distinct buyer from table) b cross join
(select distinct product p from table) p left join
table t
on t.buyer = b.buyer and t.product = p.product;
EDIT II:
If you have NULL values for buyer and/or product, then use NULL safe comparisons:
select b.buyer, p.product, coalesce(t.qty, 0) as qty
from (select distinct buyer from table) b cross join
(select distinct product p from table) p left join
table t
on t.buyer is not distinct from b.buyer and
t.product is not distinct from p.product;
(As a minor side note: I really do not like the use of distinct in this construct. Why did Postgres (ANSI ?) give it such a complicated name?)

Answer 2

The solution of #Gordon is almost full, I edit like this:
declare #tb table (buyer varchar(150), product varchar(150), quantity int)
insert into #tb
values('tom','skirt',2),
('anna','skirt',3),
('tom','jeans',5)
select *
from #tb a
left join( select
distinct(product)
from #tb) b on a.product = a.product
select b.buyer, p.p, isnull(t.quantity,0)
from (select distinct buyer from #tb) b cross join
(select distinct product p from #tb) p left join
#tb t
on t.buyer = b.buyer and t.product = p.p
--where t.buyer is null
Try it.