插入包含来自两个不同SELECT查询的两个字段的行？

Question

I have the following PostgreSQL database laid out:

Now, I have trouble forming an insertion statement. I want to insert rows to DiscussionParticipation so that for each Discussion there would be two entries:

• One for the User who owns the related Activity

• One for the User who owns the Response related to the Activity

For example, if there were the following entries:

Discussion:
id: 1 | Activity_id: 1

Activity:
id: 1 | User_id: 1

Response:
id: 1 | Activity_id: 1 | User_id: 2

User:
id: 1 | username: Adam
id: 2 | username: Bill

In other words: User 'Adam' has created an activity in which user 'Bill' has created a response to. There exists a related Discussion for the activity.

Now, I'd like to add these two rows to the DiscussionParticipation table:

DiscussionParticipation:
User_id: 1 | Discussion_id: 1
User_id: 2 | Discussion_id: 1

How do I form this INSERT?

Answer 1

A list of users can participate activity of discussion union a list of users can participate response of discussion.

insert into DiscussionParticipation (User_id, Discussion_id) (

    select a.User_id as User_id, d.id as Discussion_id
    from Discussion d
    inner join Activity a on (a.id = d.Activity_id)

    union

    select r.User_id as User_id, d.id as Discussion_id
    from Discussion d
    inner join Response r on (r.Activity_id = d.Activity_id)

);