colin is awesome

CREATE TABLE voter (
email_id INTEGER,
voter_id1 INTEGER,
voter_id2 INTEGER
)

INSERT INTO voter VALUES (1, NULL, 1);
INSERT INTO voter VALUES (2, 2, NULL);
INSERT INTO voter VALUES (3, NULL, NULL);
INSERT INTO voter VALUES (4, 4, NULL);
INSERT INTO voter VALUES (5, NULL, 5);

SELECT * FROM voter;
##################################
email_id | voter_id1 | voter_id2
----------+-----------+-----------
1 | | 1
2 | 2 |
3 | |
4 | 3 |
5 | | 4
(5 rows)

SELECT
email_id,
voter_id1,
voter_id2,
CASE WHEN voter_id1 IS NULL
THEN voter_id2
ELSE voter_id1
END AS id_union
FROM
voter
WHERE
voter_id1 IS NOT NULL
OR voter_id2 IS NOT NULL;
#############################################
email_id | voter_id1 | voter_id2 | id_union
----------+-----------+-----------+----------
1 | | 1 | 1
2 | 2 | | 2
4 | 3 | | 3
5 | | 4 | 4
(4 rows)