problem join

having problem joining these correctly:

schedule
- cal_id references calendar not null
- usr_id references users not null
= unique( calZ_id, usr_id )
- result_no not null

activity
- cal_id references calendar not null
- usr_id references users not null
= unique( cal_id, usr_id )
- from_ts timestamp not null

trying to join
- cal_id
- usr_id
- schedule.result_no (might be null)
- activity.from_ts (might be null)
where schedule.usr_id = activity.usr_id would result in one row

thanks for insight!

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