MySQL 5.7

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CREATE TABLE t (`id` int, `C1` varchar(1), `C2` varchar(4));
    
INSERT INTO t (`id`, `C1`, `C2`) VALUES
    (1, 'a', NULL),
    (2, 'b', NULL),
    (3, 'c', NULL),
    (4, 'c', '3'),
    (5, 'c', '5'),
    (6, 'd', NULL);
SELECT *
FROM t
WHERE (
    C2 = 3
) OR (
    C2 IS NULL AND NOT EXISTS (
        SELECT 1
        FROM t AS x
        WHERE C1 = t.C1 AND C2 = 3
    )
)


id
C1
C2


1
a
null

2
b
null

4
c
3

6
d
null

id	C1	C2
1	a	null
2	b	null
4	c	3
6	d	null