Conditional return values in SQL - sql

Question

I have started a simple auction system where each row contains the following data:
Type:= BID|WIT
ProductName := Str
User := Str
Value := Decimal
CreatedAt := datetime
*Here WIT means withdraw, aka quit from item auction *
A user can naturally do multiple bid requests, always raising the bid value (this is handled at language level)
I would like to list all top bids from all users but under a condition, only if they are not before a WITHDRAW request for the given item.
For example, given the entries
BID,Doll,John,10,2021-11-26 10:10
BID,Ball,John,12,2021-11-26 10:11
BID,Doll,Mary,12,2021-11-26 10:12
BID,Doll,Alice,13,2021-11-26 10:13
BID,Doll,Mary,14,2021-11-26 10:14
BID,Doll,Alice,17,2021-11-26 10:14
BID,Ball,Mary,14,2021-11-26 10:14
WIT,Doll,John,00,2021-11-26 10:16
BID,Doll,Mary,20,2021-11-26 10:18
BID,Ball,John,15,2021-11-26 10:20
If I magic_sql_query(Doll) I would like to get
BID,Doll,Alice,17,2021-11-26 10:14
BID,Doll,Mary,20,2021-11-26 10:18
Also If I magic_sql_query(Ball) I would like to get:
BID,Ball,Mary,14,2021-11-26 10:14
BID,Ball,John,15,2021-11-26 10:20
How can I do it in a SQL Statement?

Answer 1

You can
use the row_number() windowing function to rank within a group (a
group is defined by the username and the product); the latest entry
gets rank 1
get rid of all entries whose rank is > 1 (i.e. the user has a later entry for this product)
get rid of all entries of type 'WIT'
with base as (select
b.type,
b.productname,
b.username,
b.value,
b.createdat,
row_number() over (partition by productname, username
order by createdat desc) as rn
from bids b
order by productname, username, createdat
)
select * from base
where rn = 1
and type = 'BID';
SQL Fiddle

Answer 2

To find all bids that have no withdrawal later (by the same user for the same product) you can use a NOT EXITS condition:
select a1.*
where a1.product_name = 'Ball'
and type = 'BID'
and not exists (select *
from auction a2
where a2.product_name = a1.product_name
and a2.user_name = a1.user_name
and a2.type = 'WIT'
and a2.created_at > a1.created_at)
Now we need to filter out the highest bids per product and user. This can be done using the dense_rank() function.
select type, product_name, user_name, value, created_at
from (
select a1.*, dense_rank() over (partition by product_name, user_name order by value desc) as rnk
from auction a1
where a1.product_name = 'Ball'
and type = 'BID'
and not exists (select *
from auction a2
where a2.product_name = a1.product_name
and a2.user_name = a1.user_name
and a2.type = 'WIT'
and a2.created_at > a1.created_at)
) t
where rnk = 1
order by created_at;
Online example

Answer 3

A left join version
select t.Type, product_name, t.User, Value, CreatedAt
from (
select b.*
, row_number() over(partition by b.product_name, b.user order by b.createdAt desc) rn
from auction b
left join auction w
on w.type = 'WIT'
and b.product_name = w.product_name
and b.user = w.user
and w.createdAt > b.createdAt
where b.type = 'BID'
and w.product_name is null
-- and b.product_name = 'Ball'
) t
where rn=1
-- and product_name = 'Ball'
order by product_name, CreatedAt;
uncomment one of .. product_name = 'Ball' to get only this product.
db<>fidlle

Answer 4

You can use MAX() window function to get for each user the last entry with type = 'BID' and the last entry with type = 'WIT'.
Then filter the results:
SELECT type, productname, "user", value, createdat
FROM (
SELECT *,
MAX(CASE WHEN type = 'BID' THEN createdat END) OVER (PARTITION BY "user") last_bid,
MAX(CASE WHEN type = 'WIT' THEN createdat END) OVER (PARTITION BY "user") last_wit
FROM tablename
WHERE productname = ?
) t
WHERE (last_wit IS NULL OR last_wit < last_bid) AND createdat = last_bid;
Replace ? with the name of the product that you want.
See the demo.