If else exist statement in select query

Here are three tables where I want a query to select which filter/s will be expired in 7 next few days.<br/>

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I have 2 columns called filterExp in the Filter table and filterExpCustom in the FilterChange table, the first is the default if the second is not empty.
All filters have an expiration value in the day terms but in some cases, some users want to change the expiration value for their own. So they can set filterExpCustom for specific customers and filters.

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Here are some actual values to better understand:<br>

Filter

INSERT [Filter] ([filterId], [filterName], [filterExp]) 
VALUES (1, N'pp', 6)
INSERT [Filter] ([filterId], [filterName], [filterExp]) 
VALUES (2, N'Carbonate', 5)
INSERT [Filter] ([filterId], [filterName], [filterExp]) 
VALUES (3, N'Carbon Block', 5)
INSERT [Filter] ([filterId], [filterName], [filterExp]) 
VALUES (4, N'Carbon Post', 12)
INSERT [Filter] ([filterId], [filterName], [filterExp]) 
VALUES (5, N'Mineral', 12)

FilterChange

INSERT [FilterChange] ([filterChnageId], [customerId], [filterId], [customerDeviceId], [filterChangeDate], [filterExpCustom]) 
VALUES (186, 3, 2, 65, CAST(N'2023-01-31' AS Date), 7)
INSERT [FilterChange] ([filterChnageId], [customerId], [filterId], [customerDeviceId], [filterChangeDate], [filterExpCustom]) 
VALUES (187, 3, 5, 65, CAST(N'2023-01-31' AS Date), NULL)
INSERT [FilterChange] ([filterChnageId], [customerId], [filterId], [customerDeviceId], [filterChangeDate], [filterExpCustom]) 
VALUES (188, 2, 3, 66, CAST(N'2023-02-01' AS Date), 10)
INSERT [FilterChange] ([filterChnageId], [customerId], [filterId], [customerDeviceId], [filterChangeDate], [filterExpCustom]) 
VALUES (189, 2, 3, 66, CAST(N'2023-02-01' AS Date), NULL)

Here is the query that I can get the expired filter/s from now until 7 days next:

CREATE PROCEDURE [dbo].[FilterExpirationGetList]
  @userName BIGINT
AS
  DECLARE @InNextDays int
  BEGIN
    SET @InNextDays = 7

    SELECT f.filterId,f.filterName, f.filterExp,
      fc.customerId, fc.customerDeviceId, fc.filterChangeDate, fc.filterExpCustom,
      c.fName, c.lName, c.cMobile, c.userName
     FROM FilterChange fc INNER JOIN
       Filter f  ON fc.filterId = f.filterId  INNER JOIN
       Customer c ON c.CustomerId = fc.customerId
     WHERE c.userName = @userName AND
      DATEADD(DAY, DATEPART(DAY, GETDATE()) 
      - DATEPART(DAY, DATEADD(DAY,f.filterExp,fc.filterChangeDate)), 
      DATEADD(DAY,f.filterExp,fc.filterChangeDate))
      BETWEEN CONVERT(DATE, GETDATE()) 
      AND CONVERT(DATE, GETDATE() + @InNextDays); 
  END

The problem with my query is I don't know how to select expired filters if filterExpCustom has some value but if it doesn't, the filterExp must be calculated in the query.

Something like this perhaps, you use ISNULL to get the correct day extension:

select *
from customer c
inner join FilterChange fc
 ON fc.customerID = fc.CustomerID
INNER JOIN Filter f
 ON f.filterID = fc.filterID
WHERE DATEADD(DAY, ISNULL(fc.filterExpCustom,filterExp), filterChangeDate) BETWEEN CAST(GETDATE() AS DATE) AND DATEADD(DAY, 7, GETDATE())

I'm not sure what you do with customers where filters expired before today though. Your design seem lacking a sort of flag that says that filterchange is "superceeded" with another change.

Alternatively you can fetch only the latest filterchange per filter type where expiration dates is less than 7 days forward. I leave it as exercise to the reader