mysql – 对几个查询的结果进行求和,然后在SQL中找到前5个

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

table: upvote
SELECT event_id, count(*) AS upvotes 
FROM upvote
GROUP BY event_id
ORDER BY upvotes DESC, rand()
LIMIT 1000

table: attending
SELECT event_id, count(*) AS attendants
FROM attending
GROUP BY event_id
ORDER BY attendants DESC, rand()
LIMIT 1000

我想组合按金额排序的所有3个查询的event_ids,然后选择前5个.我该怎么做？

编辑：这是我发生了什么：

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

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

UNION ALL
(SELECT event_id, count(*) as amount
FROM   attending
GROUP  BY event_id
ORDER  BY amount DESC, rand()
LIMIT  1000)
) x
GROUP  BY 1
ORDER  BY  sum(amount) DESC
LIMIT  5;

(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,因此不会删除重复项.

如果要为每个event_id添加计数,则此查询应执行此操作：

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;

这里棘手的部分是并非所有三个基本查询都会出现每个event_id.因此,您必须注意JOIN不会完全丢失行,并且添加不会变为NULL.

使用UNION ALL,而不是UNION.您不想删除相同的行,而是想要添加它们.

x是AS x的简写 – 表别名.子查询需要具有名称.这里可以是任何其他名称.

SOL-feature FULL OUTER JOIN没有在MySQL中实现(我上次看过),所以你必须要使用UNION. FULL OUTER JOIN将加入所有三个基本查询而不会丢失行.

回答后续问题

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;

或者,如果您想以多种方式使用基本计数：

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;