Select statement to show the corresponding user with the lowest/highest amount? - sql

Question

I want to write a select statement output that, among other things, has both a lowest_bid and highest_bid column. I know how to do that bit, but want I also want is to show the user (user_firstname and user_lastname combined into their own column) as lowest_bidder and highest_bidder. What I have so far is:
select item_name, item_reserve, count(bid_id) as number_of_bids,
min(bid_amount) as lowest_bid, ???, max(big_amount) as highest_bid,
???
from vb_items
join vb_bids on item_id=bid_item_id
join vb_users on item_seller_user_id=user_id
where bid_status = ‘ok’ and
item_sold = ‘no’
sort by item_reserve
(The ???'s are where the columns should go, once I figure out what to put there!)

Answer 1

This seems like good use of window functions. I've assumed a column vb_bids.bid_user_id. If there's no link between a bid and a user, you can't answer this question
With x as (
Select
b.bid_item_id,
count(*) over (partition by b.bid_item_id) as number_of_bids,
row_number() over (
partition by b.bid_item_id
order by b.bid_amount desc
) as high_row,
row_number() over (
partition by b.bid_item_id
order by b.bid_amount
) as low_row,
b.bid_amount,
u.user_firstname + ' ' + u.user_lastname username
From
vb_bids b
inner join
vb_users u
on b.bid_user_id = u.user_id
Where
b.bid_status = 'ok'
)
Select
i.item_name,
i.item_reserve,
min(x.number_of_bids) number_of_bids,
min(case when x.low_row = 1 then x.bid_amount end) lowest_bid,
min(case when x.low_row = 1 then x.username end) low_bidder,
min(case when x.high_row = 1 then x.bid_amount end) highest_bid,
min(case when x.high_row = 1 then x.username end) high_bidder
From
vb_items i
inner join
x
on i.item_id = x.bid_item_id
Where
i.item_sold = 'no'
Group By
i.item_name,
i.item_reserve
Order By
i.item_reserve
Example Fiddle

Answer 2

In order to get the users, I broke out the aggregates into their own tables, joined them by the item_id and filtered them by a derived value that is either the min or max of bid_amount. I could have joined to vb_bids for a third time, and kept the aggregate functions, but that would've been redundant.
This will fail if you have two low bids of the exact same amount for the same item, since the join is on bid_amount. If you use this, then you'd want to created an index on vb_bids covering bid_amount.
select item_name, item_reserve, count(bid_id) as number_of_bids,
low_bid.bid_amount as lowest_bid, low_user.first_name + ' ' + low_user.last_name,
high_bid.bid_amount as highest_bid, high_user.first_name + ' ' + high_user.last_name
from vb_items
join vb_bids AS low_bid on item_id = low_bid.bid_item_id
AND low_bid.bid_amount = (
SELECT MIN(bid_amount)
FROM vb_bids
WHERE bid_item_id = low_bid.bid_item_id)
join vb_bids AS high_bid on item_id = high_bid.bid_item_id
AND high_bid.bid_amount = (
SELECT MAX(bid_amount)
FROM vb_bids
WHERE bid_item_id = high_bid.bid_item_id)
join vb_users AS low_user on low_bid.user_id=user_id
join vb_users AS high_user on high_bid.user_id=user_id
where bid_status = ‘ok’ and
item_sold = ‘no’
group by item_name, item_reserve,
low_bid.bid_amount, low_user.first_name, low_user.last_name,
high_bid.bid_amount, high_user.first_name, high_user.last_name
order by item_reserve

Answer 3

I am a big fan of using Common Table Expressions (CTEs) for situations like this, because of the following advantages:
Separating different parts of the logic, adding to readability, and
Reducing complexity (for example, the need to GROUP BY a large number of fields, or to repeat the same join multiple times.)
So, my suggested approach would be something like this:
-- semi-colon must precede CTE
;
-- collect bid info
WITH item_bids AS (
SELECT
i.item_id, i.item_name, i.item_reserve, b.bid_id, b.bid_amount,
(u.first_name + ' ' + u.last_name) AS bid_user_name
FROM vb_items i
JOIN vb_bids b ON i.item_id = b.bid_item_id
JOIN vb_users u ON b.user_id = u.user_id
WHERE b.bid_status = 'ok'
AND i.item_sold = 'no'
),
-- group bid info
item_bid_info AS (
SELECT item_id, item_name, item_reserve
COUNT(bid_id) AS number_of_bids, MIN(bid_amount) AS lowest_bid, MAX(bid_amount) AS highest_bid
FROM item_bids
GROUP BY item_id, item_name, item_reserve
)
-- assemble final result
SELECT
bi.item_name, bi.item_reserve, bi.number_of_bids,
bi.low_bid, low_bid.bid_user_name AS low_bid_user,
bi.high_bid, high_bid.bid_user_name AS high_bid_user
FROM item_bid_info bi
JOIN item_bids AS low_bid ON bi.lowest_bid = low_bid.bid_amount AND bi.item_id = low_bid.bid_item_id
JOIN item_bids AS high_bid ON bi.lowest_bid = high_bid.bid_amount AND bi.item_id = high_bid.bid_item_id
ORDER BY bi.item_reserve;
Note that the entire SQL statement (from the starting WITH all the way down to the final semi-colon after the ORDER BY) is a single statement, and is evaluated by the optimizer as such. (Some people think each part is evaluated separately, like temp tables, and then all the rows are joined together at the end in a final step. That's not how it works. CTEs are just as efficient as sub-queries.)
Also note that this approach does a JOIN on the bid amount, so if there are identical bids for a single item, it will fail. (Seems like that should be an invalid state anyway, though, right?) Also you may have efficiency concerns depending on:
The size of your table
Whether the lookup can use an index
You could address both issues by including a unique constraint (which has the added advantage of indexing the foreign key bid_item_id as well; always a good practice):
ALTER TABLE [dbo].[vb_bids] ADD CONSTRAINT [UK_vbBids_item_amount]
UNIQUE NONCLUSTERED (bid_item_id, bid_amount)
GO
Hope that helps!