CASE Statement with condition - sql-server

Question

I think this will be easier to show an example first and then explain:
SELECT P.ID,
(CASE WHEN PC.NewCostPrice IS NULL
THEN P.Cost ELSE MAX(PC.Date) PC.NewCostPrice
END)
FROM price AS P
LEFT OUTER JOIN priceChange as PC
ON P.ID = PC.ID
So in the example, if the NewCostPrice IS NULL, meaning there wasn't a price change, then I want the normal cost (P.Cost). However, if there was a price change, I want the most recent (MAX(Date)) price change. I am not sure how to incorporate that into the CASE statement.
I feel like it can be done with a subquery and having clause but that didn't really work out when I tried. Any suggestions?
Thanks!

Answer 1

There are 2 approaches you might consider - I would test both to see which performs better for your situation.
Use ROW_NUMBER() in subquery to find most recent price change of all price changes, then join that to prices to get correct price.
Use correlated subquery (many ways of this, either in SELECT as in other answer or with OUTER APPLY) to get only most recent price change for each row of prices
If your price table is very large and you are getting a large number of prices at once, method #1 will likely be better so the correlated subquery doesn't run for every single row of the result set.
If your final query pulls back a relatively small number of records instead of huge result sets for your server, then the correlated subquery could be better for you.
1. The ROW_NUMBER() approach
SELECT
P.ID,
COALESCE(PC.NewCostPrice, P.Cost) AS LatestPrice
FROM Price AS P
LEFT OUTER JOIN (
SELECT
ID,
ROW_NUMBER() OVER (PARTITION BY ID
ORDER BY [Date] DESC) AS RowId,
NewCostPrice
FROM PriceChange
) PC
ON P.ID = PC.ID
AND PC.RowId = 1 -- Only most recent
2a. Correlated subquery (SELECT)
SELECT
P.ID,
COALESCE((
SELECT TOP 1
NewCostPrice
FROM PriceChange PC
WHERE PC.ID = P.ID
ORDER BY PC.[Date] DESC
), P.Cost) AS LatestPrice
FROM Price AS P
2b. Correlated subquery with OUTER APPLY
SELECT
P.ID,
COALESCE(PC.NewCostPrice, P.Cost) AS LatestPrice
FROM Price AS P
OUTER APPLY (
SELECT TOP 1
NewCostPrice
FROM PriceChange PC
WHERE PC.ID = P.ID
ORDER BY PC.[Date] DESC
) PC
Whether you use 2a or 2b is more likely a preference in how you want to maintain the query going forward.

Answer 2

Easy way
SELECT distinct P.ID,
ISNULL((SELECT TOP 1 PC1.NewCostPrice FROM priceChange as PC1 WHERE PC1.ID = p.id ORDER BY PC1.Date DESC), p.cost)
FROM price AS P

Answer 3

Here I assume PC.ID is not a primary key, or it makes no sense to join with ID while there could be different price on the same item.
From your query I assume you just want to fetch the latest NewCostPrice sorted by Date, by joining priceChange
SELECT
P.ID,
CASE
WHEN PC.NewCostPrice IS NULL THEN P.Cost
ELSE PC.NewCostPrice
END AS NewPrice
FROM
price AS P
LEFT JOIN
(SELECT *, RANK() OVER (PARTITION BY ID ORDER BY [Date] DESC) as rk FROM priceChange) PC ON P.ID = PC.ID AND PC.rk = 1

Answer 4

SELECT P.ID
,(CASE
WHEN PC.NewCostPrice IS NULL
THEN P.Cost
ELSE (SELECT TOP 1 PC1.NewCostPrice
FROM priceChange PC1
WHERE PC1.ID = PC.ID
GROUP BY PC1.NewCostPrice, PC1.Date
ORDER BY PC1.Date DESC
)
END
)
FROM price AS P
LEFT OUTER JOIN priceChange as PC
ON P.ID = PC.ID