SQL CASE switch for parameter - sql

Question

I have a parameter that can be 1 of 3 possible values 0, 2 or NULL (these are values for an enum 0 is Pending, 1 is UnderReview and 2 is Closed). If it is NULL it should retrieve all regardless of the Status but If it is 2 it should retrieve all the closed ones however if the value is 0 it should retrieve the ones that are pending or under review. This last one means that I want to retrieve records that have a status of either 0 or 1. I have the following code so far and tried with a CASE switch but it doesn't work.
SELECT * FROM PurchaseOrder
WHERE (CreationDate >= #StartDate AND CreationDate <= #EndDate)
AND (#POStatus IS NULL OR [Status] = #POStatus)
AND (#POStatus IS NULL OR [Status] = CASE #POStatus WHEN 0 THEN 1 END)
AND (#PurchaseOrderIdSearch IS NULL OR PurchaseOrderId LIKE #PurchaseOrderIdSearch)
AND EmployeeId = #EmployeeId
ORDER BY CreationDate DESC, PurchaseOrderId DESC
I solved it adding an if statement, I was looking for a simpler way to do it to avoid repetition, this is how it looks like now:
IF #POStatus = 0
BEGIN
SELECT * FROM PurchaseOrder
WHERE (CreationDate >= #StartDate AND CreationDate <= #EndDate)
AND (([Status] = #POStatus)
OR ([Status] = 1))
AND (#PurchaseOrderIdSearch IS NULL OR PurchaseOrderId LIKE #PurchaseOrderIdSearch)
AND EmployeeId = #EmployeeId
ORDER BY CreationDate DESC, PurchaseOrderId DESC
END
ELSE
BEGIN
SELECT * FROM PurchaseOrder
WHERE (CreationDate >= #StartDate AND CreationDate <= #EndDate)
AND (#POStatus IS NULL OR [Status] = #POStatus)
AND (#PurchaseOrderIdSearch IS NULL OR PurchaseOrderId LIKE #PurchaseOrderIdSearch)
AND EmployeeId = #EmployeeId
ORDER BY CreationDate DESC, PurchaseOrderId DESC
END

Answer 1

I suppose you're looking for something like
DECLARE #MyParam INT; --Try to set 0, 1 or 2
SELECT *
FROM
(
VALUES
(0),
(1),
(2)
) T(Value)
WHERE (Value = #MyParam) OR (#MyParam IS NULL);
Here is a db<>fiddle to see how it's working.
Update:
It seems like you're looking for
DECLARE #MyParam INT; --Try to set 0, 1 or 2
SELECT *
FROM
(
VALUES
(0),
(1),
(2)
) T(Value)
WHERE
(
CASE WHEN #MyParam = 0 OR #MyParam = 1 THEN 1 ELSE 0 END = 1
AND Value IN(0, 1)
)
OR
(Value = #MyParam AND #MyParam = 2)
OR
#MyParam IS NULL;
Here is a db<>fiddle