Selecting from where column equals a value or is null with cases (Mssql)

Question

In the query below im doing a search on a table called welds, this table has a column called Welds.Prefix which has NULL rows, rows with actual values, and rows with an empty value. I need help modifying my query below so that when the user inputs a non null and non empty value in @Prefix it will find the rows that match. And when @Prefix is blank it will find where Welds.Prefix is blank or where Welds.Prefix is null but im not sure how to go about it, I understand that I could do an update query and change the columns with null values to blank spaces but im looking for another way. I run the query below and I get an error "Incorrect syntax near the keyword 'OR'" I understand why im getting the error but im not sure how to fix it, thanks.

Note: The @Prefix will be inputted on the front end and will not be bound to just '', I just put it in the query for questions sake.

@Prefix varchar(30) = ''

select * 
from welds
WHERE Welds.Prefix = CASE WHEN @Prefix <> '' THEN @Prefix END
          OR Welds.Prefix = CASE WHEN @Prefix = '' THEN '' OR IS NULL END

EDIT: I basically want to selct rows where Weld.Prefix is null and where Weld.Prefix = ' ' when @Prefix = ' '

Answer 1

I think this is the logic you want:

SELECT * 
FROM welds
WHERE Welds.Prefix = @sPrefix OR
      (@sPrefix = '' AND NULLIF(Welds.Prefix, '') IS NULL)