SQL Server 2017

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CREATE TABLE TEMP
(ID int, Name NVARCHAR(100),  Cell NVARCHAR(100) NULL)

INSERT INTO TEMP(ID,Name, Cell)
VALUES(1,'John','123'), (2,'Sally',NULL)
2 rows affected

DECLARE @Cell NVARCHAR(100) = NULL
Select * from Temp
WHERE Cell = @Cell OR COALESCE(Cell, @Cell) IS NULL;


ID
Name
Cell


2
Sally
null


DECLARE @Cell NVARCHAR(100) = '123'
Select * from Temp
WHERE Cell = @Cell OR COALESCE(Cell, @Cell) IS NULL;


ID
Name
Cell


1
John
123