Question

For example, I have a table Person having an ID for each entry and I have a table PersonEvent which records the personID of each person at an event.

If PersonEvent contains no rows for the event, everyone in the Person table attended. However if there are entries in the PersonEvent table, only those people attended the event.

I want to do a query such as:

SELECT * 
FROM Person p 
WHERE ((SELECT COUNT(*)
        FROM PersonEvent pe 
        WHERE pe.personID = p.ID 
        AND pe.eventID = '290') = 0 
      OR EXISTS 
        (SELECT * 
         FROM PersonEvent pe 
         WHERE pe.personID = p.ID 
         AND pe.eventID = '290'))

How do I do this?

Answer 1

SELECT *
  FROM Person p
 WHERE NOT EXISTS ( -- not a single person for the event
       SELECT *
         FROM PersonEvent
        WHERE eventID = '290')
    OR EXISTS ( -- the person for the event
       SELECT *
         FROM PersonEvent pe
        WHERE pe.personID = p.ID AND pe.eventID = '290')