Case Statement in SQL WHERE Clause

DECLARE @FirstName VARCHAR(100)

SET @FirstName = ''

 

DECLARE @LastName VARCHAR(100)

SET @LastName = 'John' 

 

SELECT FirstName, LastName

FROM Contacts

WHERE  

    FirstName = CASE

    WHEN LEN(@FirstName) > 0 THEN  @FirstName

    ELSE FirstName

    END

AND

    LastName = CASE

    WHEN LEN(@LastName) > 0 THEN  @LastName

    ELSE LastName

    END

GO