Postgres 13

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CREATE TABLE domains_1 (domain varchar);
CREATE TABLE domains_2 (domain varchar);

INSERT INTO domains_1 VALUES ('example_1.com'), ('example_2.com');
INSERT INTO domains_2 VALUES ('example_2.com'), ('example_3.com');
2 rows affected

2 rows affected

-- "unique domains"
SELECT domain
FROM   domains_1
FULL   JOIN domains_2 USING (domain);

domain

example_1.com

example_2.com

example_3.com

-- domains without match in other table
SELECT domain
FROM   domains_1 a
FULL   JOIN domains_2 b USING (domain)
WHERE  a.domain IS NULL OR b.domain IS NULL;

domain

example_1.com

example_3.com

-- same:
(TABLE domains_1 EXCEPT TABLE domains_2)
UNION ALL
(TABLE domains_2 EXCEPT TABLE domains_1)

domain

example_1.com

example_3.com