SQL Server 2017

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CREATE TABLE mytable (id int, val INT);

INSERT INTO mytable
VALUES (1,'25'),
       (2,'55'),
       (3,'25'),
       (4,'10'),
       (5,'60');
5 rows affected

DECLARE @input INT = NULL

SELECT *
FROM mytable
WHERE (@input IS NULL OR id = @input)


id
val


1
25

2
55

3
25

4
10

5
60


DECLARE @input INT = 2

SELECT *
FROM mytable
WHERE (@input IS NULL OR id = @input)


id
val


2
55