SQL query to allow NULL for a highest_bid column when no bid has been placed yet - sql

Question

For school I need to make a function on an auction website. For this I need to join a couple of tables in a VIEW. This worked just fine, until I needed to add a filter for price range. Seems easy enough but the query result needs to allow a NULL when no bid has been placed.
The Statement for the View:
SELECT I.itemID, I.title, I.startPrice, B.highestBid, Cfi.category, I.endDate
FROM dbo.Items AS I INNER JOIN dbo.category_for_item AS Cfi ON V.itemID = Vir.itemID
LEFT OUTER JOIN dbo.Bid AS B ON V.itemID = B.itemID
This would get the following Table:
itemID title startPrice highestBid category endDate
1 1234 Alfa 25 26 PC 2018-09-22
2 1234 Alfa 25 NULL PC 2018-09-22
3 5678 Bravo 9 20 Console 2018-07-03
4 5678 Bravo 9 15 Console 2018-07-03
5 5678 Bravo 9 NULL Console 2018-07-03
6 9876 Charlie 84 100 Stamps 2018-06-14
7 9876 Charlie 84 90 Stamps 2018-06-14
8 9876 Charlie 84 85 Stamps 2018-06-14
9 9876 Charlie 84 NULL Stamps 2018-06-14
10 1470 Delta 98 100 Fashion 2018-06-15
11 1470 Delta 98 99 Fashion 2018-06-15
12 1470 Delta 98 NULL Fashion 2018-06-15
13 9631 Echo 56 65 Cars 2018-06-25
14 9631 Echo 56 NULL Cars 2018-06-25
15 7856 Foxtrot 98 NULL Dolls 2018-12-26
After looking around for answers I got a query for joining the VIEW on itself with only showing the highest bid instead of all bids:
SELECT VW.itemID, VW.title, VW.startPrice, VW.highestBid, VW.category, VW.endDate
FROM VW_SEARCH AS VW
INNER JOIN (SELECT itemID, MAX(highestBid) AS MaxBid
FROM VW_SEARCH
GROUP BY itemID) VJ
ON VW.itemID = VJ.itemID AND VW.highestBid = VJ.MaxBid
This gave the next results:
itemID title startPrice highestBid category endDate
1 1234 Alfa 25 26 PC 2018-09-22
2 5678 Bravo 9 20 Console 2018-07-03
3 9876 Charlie 84 85 Stamps 2018-06-14
4 1470 Delta 98 100 Fashion 2018-06-15
5 9631 Echo 56 65 Cars 2018-06-25
As I expected the result only showed the items with at least one bid on them. I tried added one extra condition on the subQuery and Joining RIGHT OUTER to make sure I would not get doubles of an itemID.
SELECT VW.itemID, VW.title, VW.startPrice, VW.highestBid, VW.category, VW.endDate
FROM VW_SEARCH AS VW
RIGHT OUTER JOIN (SELECT itemID, MAX(highestBid) AS MaxBid
FROM VW_SEARCH
WHERE highestBid > 0 OR highestBid IS NULL
GROUP BY itemID) VJ
ON VW.itemID = VJ.itemID AND VW.highestBid = VJ.MaxBid
This gave the following results (did not add result 5 - 1199 because it is all the same as result 4, this would happen in the actual table not the example table from above):
itemID title startPrice highestBid category endDate
1 1234 Alfa 25 26 PC 2018-09-22
2 5678 Bravo 9 20 Console 2018-07-03
3 9876 Charlie 84 85 Stamps 2018-06-14
4 NULL NULL NULL NULL NULL NULL
1200 1470 Delta 98 100 Fashion 2018-06-15
1201 9631 Echo 56 65 Cars 2018-06-25
While this is technicly allowing a NULL in the colums I need to get a result in the likes of :
itemID title startPrice highestBid catgory endDate
1 1234 Alfa 25 26 PC 2018-09-22
2 5678 Bravo 9 20 Console 2018-07-03
3 9876 Charlie 84 85 Stamps 2018-06-14
4 1470 Delta 98 100 Fashion 2018-06-15
5 9631 Echo 56 65 Cars 2018-06-25
6 7856 Foxtrot 98 NULL Dolls 2018-12-26
How do I get the desired result, or is it just impossible?
Also if the query could be written better, please say so.
Thanks in advance.

Answer 1

Solve the problem using a left join:
SELECT VW.itemID, VW.title, VW.startPrice, VW.highestBid, VW.category, VW.endDate
FROM VW_SEARCH VW LEFT JOIN
(SELECT itemID, MAX(highestBid) AS MaxBid
FROM VW_SEARCH
GROUP BY itemID
) VJ
ON VW.itemID = VJ.itemID AND VW.highestBid = VJ.MaxBid;
Or, use the ANSI-standard ROW_NUMBER() function:
select vw.*
from (select vw.*,
row_number() over (partition by itemID
order by highestBid nulls last
) as seqnum
from vw_search vw
) vw
where seqnum = 1;
This guarantees one row per item.
Note: Not all databases support NULLS LAST. This may not even be necessary, but you can also implement it using a case expression.

Answer 2

Can you give the definition of the view at least? Maybe the table definition too.

Answer 3

I would go only with subquery as because identity column :
select vw.*
from vw_search vw
where id = (select vm1.id
from vw_search vm1
where vm.itemID = vw1.itemID and vm1.highestBid is not null
order by vm1.highestBid desc
limit 1
);
However, some DBMS has not support LIMIT clause such as SQL Server if so, then you can use TOP clause instead.