I think you need to distinguish which table your ID and post_id come from. Take a look below and notice how the tables get a variable assigned. You could also just use the table name, but this reads easier.
SELECT P.post_id FROM [prefix]_calp_events as EV
JOIN [prefix]_posts as P ON EV.ID = P.post_id
You stated that you want to read the results from both tables. The join actually returns only matching records in both tables. Check out https://www.w3schools.com/sql/sql_join.asp for details on how joins work.
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