How to return the correspondant date of a Maximum value - sql

Question

I have a table MyTable like this:
Date Qty Price
2017-07-01 2 5.00
2017-07-08 3 4.00
2017-08-08 1 6.00
If I do a query to get the Maximum total cost like this:
SELECT max(Qty * Price) FROM MyTable
I get 12.00 which is correct.
How can I have the corresponding Date which is 2017-07-08?
In case of several equal Maximum values, I would like to have the smallest (oldest) date.

Answer 1

create table MyTable (date datetime, qty int, price int)
go
insert into MyTable(date,qty,price)
values
('2017-07-01', 2, 5.00),
('2017-07-08', 3, 4.00),
('2017-08-08', 1, 6.00);
with q as
(
select *, row_number() over (order by qty*price desc) rn
from MyTable
)
select date, qty, price
from q
where rn = 1;
outputs
date qty price
----------------------- ----------- -----------
2017-07-08 00:00:00.000 3 4
(1 row(s) affected)

Answer 2

If you need exactly one row with max Qty * Price, oldest day
SELECT TOP(1) *
FROM MyTable
ORDER BY Qty * Price DESC, [Date];
If you need all the rows with the same max Qty * Price value
SELECT TOP(1) WITH TIES *
FROM MyTable
ORDER BY Qty * Price DESC;

Answer 3

Just for the fun of it...
SELECT
[Date] = CAST(SUBSTRING(mv.MaxBin, 9, 8) AS DATE),
Qty = CAST(SUBSTRING(mv.MaxBin, 17, 4) AS INT),
Price = CAST(SUBSTRING(mv.MaxBin, 21, 8) AS MONEY)
FROM (
SELECT
MaxBin = MAX(CAST(td.Qty * td.Price AS BINARY(8)) + CAST(td.Date AS BINARY(8)) + CAST(td.Qty AS BINARY(4)) + CAST(td.Price AS BINARY(8)))
FROM
#TestData td
) mv;
Edit: How and why this works...
As you notes in the opening post, “SELECT max(Qty * Price) FROM MyTable” gets you the correct value but that doesn’t help you get the remaining columns from that row.
So the idea is to do the calculation, and then concatenate the other column values to the calculated results so that we can get all the desired columns using the previous logic [MAX(Qty * Price).
Casting things to fixed length binary values allows us to do just that. We know that Qty * Price = ToalPrice or in terms of data types INT * MONEY = MONEY, which is an 8 byte data type… So I know that I can cast any $ amount as a BINARY(8) and know 1) that I’ll always be able to convert it back to money later, 2) the binary value is EXACTLY 8 characters long and 3) I can sort the binary as I would the original value.
Same goes for Date which is a date data type which is 3 bytes long (Yea, I used 8 instead of 3… I’m used to the DATETIME type… my bad on that one)… and so on, with the other columns.
So, by concatenating the binary values together, I can table the MAX of those values, just like you did with max(Qty * Price) but now I have the entire row of data. Now all that has to be done if break the concatenated binary value back into its constituent parts (which is easy because we know exactly how long each section is) using the SUBSTRING function and the cast them back into their original data types.