MySQL 8.0

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select version();


version()


8.0.27


CREATE TABLE urls(url VARCHAR(20), `date` VARCHAR(20));

INSERT INTO urls VALUES
('http://url1',	'a'),
('http://url2',	'b'),
('http://url3',	'c'),
('http://url4',	'd');
CREATE TABLE tags(url VARCHAR(20), tags Varchar(20));

INSERT INTO tags VALUES
('http://url1',	'x'),
('http://url1',	'y'),
('http://url2',	'x'),
('http://url2',	'y'),
('http://url3',	'x'),
('http://url3',	'z'),
('http://url4',	'z');
SELECT
    urls.url,
    GROUP_CONCAT(t1.tags) as "similar tags"
FROM urls
INNER JOIN tags t1 ON t1.url=urls.url
INNER JOIN tags t2 ON t2.url='http://url1' and t2.tags = t1.tags
GROUP BY urls.url


url
similar tags


http://url1
y,x

http://url2
y,x

http://url3
x


SELECT
    urls.url,
    GROUP_CONCAT(t1.tags) as "similar tags"
FROM urls
INNER JOIN tags t1 ON t1.url=urls.url
INNER JOIN tags t2 ON t2.url='http://url4' and t2.tags = t1.tags
GROUP BY urls.url


url
similar tags


http://url3
z

http://url4
z