SQL - update each row of a column using while loop - sql-server

Question

My original table looks like:
week volume cost
1 11 null
2 32 null
3 80 null
4 75 null
5 50 null
...
51 28 null
I want to update the cost field by applying a more intelligent rule as follows:
if volume < 13, then use a rateA (loose shipment price)
if volume >= 13 and < 25, then use a rateB (20' container price)
if volume >= 25 and < 45, then use a rateC (40' container price)
I want to get the lowest cost by flexibly using above 3 different rates according to the "volume". For instance, in week 4, the "REMAINING VOLUME" initially is 75, I should apply one 40'container cost to load a portion of the volume. Then the "REMAINING VOLUME" is 30, I should apply a 20'container cost to load a portion of the rest. Then the "REMAINING VOLUME" is 5, I should apply the loose shipment price, keep doing in such way until "REMAINING VOLUME" = 0. By doing so it will give me the best combination to minimise the cost per week. Therefore, a while loop needs to be applied to the "REMAINING VOLUME" which can give indication about how to choose the different rates.
the final updated table should look like:
week volume cost
1 11 rateA
2 32 rateB + rateA
3 80 rateC + rateB + rateA
4 75 rateC + rateB + rateA
5 50 rateC + rateA
...
51 28 rateB + rateA

Answer 1

If the cases are fixed and the number of them is small, the following approach works perfectly fine (with SQL Server, anyway; I don't know about other DB's -- see note below) ...
UPDATE #Orders
SET [Cost] = CASE
WHEN 0 * 44 + 0 * 24 + 1 * 12 >= [Volume] THEN 'rateA'
WHEN 0 * 44 + 1 * 24 + 0 * 12 >= [Volume] THEN 'rateB'
WHEN 0 * 44 + 1 * 24 + 1 * 12 >= [Volume] THEN 'rateB + rateA'
WHEN 1 * 44 + 0 + 24 + 0 * 12 >= [Volume] THEN 'rateC'
WHEN 1 * 44 + 0 + 24 + 1 * 12 >= [Volume] THEN 'rateC + rateA'
WHEN 1 * 44 + 1 + 24 + 0 * 12 >= [Volume] THEN 'rateC + rateB'
WHEN 1 * 44 + 1 + 24 + 1 * 12 >= [Volume] THEN 'rateC + rateB + rateA'
ELSE 'too big'
END
FROM #Orders
;
No looping needed!
Notice the straightforward progression of the cases; you can extend the pattern to include larger weights if you want.
This works with SQL Server because you are guaranteed that the first matching condition is used, so if it matches the third WHEN case it won't try to evaluate the fourth, fifth, etc. I don't know if this is true for other databases.