计算每种活动类型的总数-Oracle-SQL

Question

I have 3 activity types and need to display how many people have taken part in each event. So for example I need the query to display:

• activity type ----------- total
• Snowboarding --------- 9
• Skiing ------------------- 7
• Sledging ---------------- 5

So far I have written the following query:

SELECT type_of_activity, COUNT (DISTINCT individual_id) 
FROM activities, events, book_activity
WHERE activities.activity_code = events.activity_code 
      AND events.event_id = book_activity.event_id;

This is obviously wrong and comes up with the error " not a single group function "

What I need it to do is count each unique instances of individual ID where type - skiing for each type. I have looked at many resources but cannot find any examples or help

Answer 1

SELECT a.type_of_activity, COUNT(ba.individual_id)
FROM BOOK_ACTIVITY ba
JOIN EVENTS e
  ON e.event_id = ba.event_id
JOIN ACTIVITIES a
  ON a.activity_code = e.activity_code
GROUP BY a.type_of_activity;