SQL查询中的多个聚合函数

Question

For this example I got 3 simple tables (Page, Subs and Followers):

For each page I need to know how many subs and followers it has. My result is supposed to look like this:

I tried using the COUNT function in combination with a GROUP BY like this:

SELECT p.ID, COUNT(s.UID) AS SubCount, COUNT(f.UID) AS FollowCount 
FROM page p, subs s, followers f 
WHERE p.ID = s.ID AND p.ID = f.ID AND s.ID = f.ID 
GROUP BY p.ID

Obviously this statement returns a wrong result.

My other attempt was using two different SELECT statements and then combining the two subresults into one table.

SELECT p.ID, COUNT(s.UID) AS SubCount FROM page p, subs s WHERE p.ID = s.ID GROUP BY p.ID

and

SELECT p.ID, COUNT(f.UID) AS FollowCount FROM page p, follow f WHERE p.ID = f.ID GROUP BY p.ID

I feel like there has to be a simpler / shorter way of doing it but I'm too unexperienced to find it.

Answer 1

Never use commas in the FROM clause. Always use proper, explicit, standard JOIN syntax.

Next, learn what COUNT() does. It counts the number of non-NULL values. So, your expressions are going to return the same value -- because f.UID and s.UID are never NULL (due to the JOIN conditions).

The issue is that the different dimensions are multiplying the amounts. A simple fix is to use COUNT(DISTINCT) :

SELECT p.ID, COUNT(DISTINCT s.UID) AS SubCount, COUNT(DISTINCT f.UID) AS FollowCount 
FROM page p JOIN
     subs s
     ON p.ID = s.ID JOIN
     followers f 
     ON s.ID = f.ID 
GROUP BY p.ID;

The inner joins are equivalent to the original query. You probably want left join s so you can get counts of zero:

SELECT p.ID, COUNT(DISTINCT s.UID) AS SubCount, COUNT(DISTINCT f.UID) AS FollowCount 
FROM page p LEFT JOIN
     subs s
     ON p.ID = s.ID LEFT JOIN
     followers f 
     ON p.ID = f.ID 
GROUP BY p.ID;

Answer 2

Scalar subquery should work in this case.

SELECT p.id,
       (SELECT Count(s_uid)
        FROM   subs s1
        WHERE  s1.s_id = p.id) AS cnt_subs,
       (SELECT Count(f_uid)
        FROM   followers f1
        WHERE  f1.f_id = p.id) AS cnt_fol
FROM   page p
GROUP  BY p.id;