来自三个表的MySQL JOIN

Question

SELECT title
     , event_date
     , event_end_date
     , individual_price
     , event_capacity
     , e.id
     , COUNT(r.event_id) registrants_count
     , COUNT(w.event_id) waiting_count 
  FROM ni803_eb_events e
  LEFT 
  JOIN ni803_eb_registrants r
    ON r.event_id = e.id
  LEFT 
  JOIN ni803_eb_waiting_lists w
    ON w.event_id = e.id
 WHERE event_capacity > 0 
 GROUP 
    BY event_date DESC;

I got three tables:

• ni803_eb_events - list of events with unique ID
• ni803_eb_registrants - list of people registered to events at every registrant there is event_id = ID from event list
• ni803_eb_waiting_lists - its just list of people who registered after event was full, same principal as registrants also there is event_id = ID of event which people registered to

I need to show ni803_eb_events where I COUNT rows from ni803_eb_registrants (id = event_id count number of same event_id) and also COUNT rows from ni803_eb_waiting_lists.

When I use just one of JOINs registrants or waiting_list it work just fine.... but not both of them :(

At screenshot below there is something wrong I marked it there are nowhere 69 registrants, there much more impossible numbers...

Answer 1

The issue is because you could get a Cartesian due to not always having records in either/or table. I suggest doing the pre-aggregates grouped by ID within each sub-table. Then those are left-joined to the original table of events. So at MOST, there would be 1 record in the registrants or 1 record in the waiting, never more. So your outermost query does not need a COUNT() or GROUP BY clause. You get each event with the respective counts.

SELECT 
      title, 
      event_date, 
      event_end_date, 
      individual_price, 
      event_capacity, 
      e.id, 
      reg.regCnt as registrants_count, 
      wait.waitCnt as waiting_count 
   FROM 
      ni803_eb_events e
         LEFT JOIN 
         ( select r.event_id, COUNT(*) regCnt
              from ni803_eb_registrants r
              group by r.event_id ) reg
            ON e.id = reg.event_id
         LEFT JOIN 
         ( select w.event_id, COUNT(*) waitCnt
              from ni803_eb_waiting_lists w
              group by w.event_id ) wait
            ON e.id = wait.event_id
   WHERE 
      e.event_capacity > 0 
   order by
      event_date DESC;