here"s the problem. For a detailed description, please see below.

1. how should inner join be used compared with separate queries? should inner join be used or should it be considered on a case-by-case basis?
2. when I have the above doubts, is there any way to experiment on my own to get the results?

the requirement goes something like this: the time to find several qualified user_id, from one table (User) and then go to another table (Post) to find the posts posted by these people.

the first way is clear, inner join:

SELECT create_time
FROM Post
WHERE user_id IN (xxx)