Hi check here I don't know how you build your table I have create a table with ID, Firstname,city and retrieving all firstname when city is matched or city is null
CREATE TABLE [dbo].[tblDemo]([id] [int] NOT NULL,[FirstName] [varchar](50) NULL,
[City] [varchar](50) NULL) ON [PRIMARY]
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this is my query to retrieve the firstname when city is null or value is passed
Select FirstName From tbldemo Where city is null or city like 'RJY'
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