Check if a certain Row existis, else select different row - sql

Question

lets say I have a table containing
|vendor | price| productID|
|--------------------------------|
|abc | 6 | 0001 |
|1 | 7 | 0001 |
|def | 8 | 0001 |
|xyz | 30 | 0002 |
|zxy | 32 | 0002 |
now I want to get the vendor which has the min() price for a product
for product 0001 that would be Vendor abc
for product 0002 that would be Vendor xyz
BUT! IF there is a Vendor named 1 I would like to see his name instead of the actual vendor with the min() price, if there is no Vendor named 1 for a product, I want to see the the one with the min() price again
if that makes any sense for you.. its kinda like a if-else construct but I dont know how to do it in SQL
(sorry for the bad formatted table, I just dont get it formatted the right way)
Thank you

Answer 1

This is a prioritization query. One method is to use row_number() and to put the rules for prioritization into the order by. This resulting query:
select t.*
from (select t.*,
row_number() over (partition by productId
order by (case when vendorid = 1 then 1 else 2 end),
price asc
) as seqnum
from t
) t
where seqnum = 1;

Answer 2

The (currently) accepted answer does not meet the OP's requirement. The requirement was to show the minimum price for each product, even if vendor '1' does not have the lowest price. However, when someone else has the lowest price but vendor '1' also sells the same product, replace the vendor name with '1' (but don't change the lowest price to vendor `1``s price). This looks like "will meet my competitors' lowest price" type of arrangement.
Query and output:
with
price_table ( vendor, price, productid ) as (
select 'abc', 6 , '0001' from dual union all
select '1' , 7 , '0001' from dual union all
select 'def', 8 , '0001' from dual union all
select 'xyz', 30 , '0002' from dual union all
select 'zxy', 32 , '0002' from dual
),
prep ( vendor, price, productid, rn, ct ) as (
select vendor, price, productid,
row_number() over (partition by productid order by price),
count( case when vendor = '1' then 1 end ) over ( partition by productid)
from price_table
)
select case when ct = 0 then vendor else '1' end as vendor,
price,
productid
from prep
where rn = 1
;
VENDOR PRICE PROD
------ ------- ----
1 6 0001
xyz 30 0002