Sum results of a few queries and then find top 5 in SQL

To UNION the resulting rows of all three queries and then pick the 5 rows with the highest amount:

(SELECT event_id, count(*) AS amount
FROM   pageview 
GROUP  BY event_id
ORDER  BY pageviews DESC, rand()
LIMIT  1000)

UNION ALL
(SELECT event_id, count(*)
FROM   upvote
GROUP  BY event_id
ORDER  BY upvotes DESC, rand()
LIMIT  1000)

UNION ALL
(SELECT event_id, count(*)
FROM   attending
GROUP  BY event_id
ORDER  BY attendants DESC, rand()
LIMIT  1000)

ORDER  BY 2 DESC
LIMIT  5;

The manual:

To apply ORDER BY or LIMIT to an individual SELECT, place the
clause inside the parentheses that enclose the SELECT.

UNION ALL to keep duplicates.

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To add the counts for every event_id:

SELECT event_id, sum(amount) AS total
FROM (
   (SELECT event_id, count(*) AS amount
    FROM   pageview 
    GROUP  BY event_id
    ORDER  BY pageviews DESC, rand()
    LIMIT  1000)
    
    UNION ALL
    (SELECT event_id, count(*)
    FROM   upvote
    GROUP  BY event_id
    ORDER  BY upvotes DESC, rand()
    LIMIT  1000)
    
    UNION ALL
    (SELECT event_id, count(*)
    FROM   attending
    GROUP  BY event_id
    ORDER  BY attendants DESC, rand()
    LIMIT  1000)
    ) x
GROUP  BY 1
ORDER  BY sum(amount) DESC
LIMIT  5;

The tricky part here is that not every event_id will be present in all three base queries. So take care that a JOIN does not lose rows completely and additions don’t turn out NULL.

Use UNION ALL, not UNION. You don’t want to remove identical rows, you want to add them up.

x is a table alias and shorthand for AS x. It is required for for a subquery to have a name. Can be any other name here.

The SOL feature FULL OUTER JOIN is not implemented in MySQL (last time I checked), so you have to make do with UNION. FULL OUTER JOIN would join all three base queries without losing rows.

Answer to follow-up question

SELECT event_id, sum(amount) AS total
FROM (
   (SELECT event_id, count(*) / 100 AS amount
    FROM   pageview ... )
    
    UNION ALL
    (SELECT event_id, count(*) * 5 
    FROM   upvote ... )
    
    UNION ALL
    (SELECT event_id, count(*) * 10
    FROM   attending ... )
    ) x
GROUP  BY 1
ORDER  BY  sum(amount) DESC
LIMIT  5;

Or, to use the base counts in multiple ways:

SELECT event_id
      ,sum(CASE source
              WHEN 'p' THEN amount / 100
              WHEN 'u' THEN amount * 5
              WHEN 'a' THEN amount * 10
              ELSE 0
           END)  AS total
FROM (
   (SELECT event_id, 'p'::text AS source, count(*) AS amount
    FROM   pageview ... )
    
    UNION ALL
    (SELECT event_id, 'u'::text, count(*)
    FROM   upvote ... )
    
    UNION ALL
    (SELECT event_id, 'a'::text, count(*)
    FROM   attending ... )
    ) x
GROUP  BY 1
ORDER  BY 2 DESC
LIMIT  5;