SQL Server 2019

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CREATE TABLE tab(a INT, b VARCHAR(10));
INSERT INTO tab(a,b) VALUES(1,'a'),(1, NULL),(NULL, 'a'),(2,'b');
4 rows affected

DECLARE @a INT;

SET @a = 1;    -- specific NOT NULL value

SELECT *
FROM tab t
WHERE EXISTS(SELECT t.a INTERSECT SELECT @a UNION SELECT @a WHERE @a = '-1');

a
b

1
a

1
null

DECLARE @a INT;

SET @a = NULL; -- NULL value

SELECT *
FROM tab t
WHERE EXISTS(SELECT t.a INTERSECT SELECT @a UNION SELECT @a WHERE @a = '-1');

a
b

null
a

DECLARE @a INT;

SET @a = -1;   -- all values

SELECT *
FROM tab t
WHERE EXISTS(SELECT t.a INTERSECT SELECT @a UNION SELECT @a WHERE @a = '-1');

a
b

1
a

1
null

null
a

2
b

a	b
1	a
1	null

a	b
1	a
1	null
null	a
2	b