SELECT a.pk
FROM table1 a, table2 b
WHERE a.pk *= b.fk
With LEFT JOIN:
SELECT posts.title,
GROUP_CONCAT(tags.tag) AS tagarr
FROM posts
LEFT JOIN post_tags ON post_tags.post_id = posts.id
LEFT JOIN tags ON tags.tag_id = post_tags.tag_id
SELECT posts.title,
GROUP_CONCAT(tags.tag) AS tagarr
FROM posts, post_tags, tags
WHERE posts.id *= post_tags.post_id
AND tags.tag_id *= post_tags.tag_id
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '= post_tags.post_id [...]The ANSI 89 syntax for inner joins is pretty ubiquitous, as it can be a little easier to write code generators that way.
https://www.postgresql.org/docs/13/tutorial-join.html
See also join_type: https://www.postgresql.org/docs/13/sql-select.html
Reason being that it's used to declare and restrict which table should be updated in the UPDATE section as opposed to optional joined tables listed in the FROM section.
PS: The *= is plateform specific however (have no clue which). Usually you just write a more SQL standard WHERE section.