Mysql - 2在一个查询中计数，第二个计数结果错误

Question

I just inherited a codebase, and there's an existing query that is incorrect. It looks like (genericized tables & columns here):

select p.user_id, p.photo_id, p.date, count(p.photo_id) as 'Photos', count(v.photo_id) as 'Views'
from photos p
LEFT OUTER JOIN userviews v on v.user_id = p.user_id and v.photo_id = p.photo_id
where p.photo_id in ( 1,2,3 [...] )
GROUP BY photo_id, user_id
ORDER BY user_id, photo_id

The result set is close to correct, so close that no one noticed it was wrong for a while.

If there's no data in table 2 (userviews), then the resulting row is correct.

However, if there is data there, the resultset is duplicating the count values from table 1, ie:

1082    3381    2012-05-25 08:50:20 3   3 <--WRONG, should be 1
1082    3387    2012-07-26 15:02:36 2   2 <--WRONG, should be 4
1117    3381    2012-05-23 03:46:02 1   0 <--CORRECT
1117    3382    2012-05-23 03:45:54 1   0 <--CORRECT
1117    3383    2012-05-23 03:45:09 1   0 <--CORRECT

Now, if this were SQL server, I'd just rewrite the damn thing using a CTE, but Mysql doesn't support a WITH clause, and I don't think I can subquery these as they return multiple values.

So how do I fix this in Mysql? Thanks in advance, I'm stumped on this one.

Answer 1

Try this query

SELECT p.user_id, p.photo_id, p.Photos, ph.date, v.Views
FROM photos ph,
      (SELECT p.user_id, p.photo_id, count(p.photo_id) as 'Photos'
      FROM photos p
      WHERE p.photo_id in ( 1,2,3 [...] )
      GROUP BY p.photo_id, p.user_id ) p,
     (SELECT v.user_id, v.photo_id, count(v.photo_id) as 'Views'
      FROM  userviews v
      WHERE v.photo_id in ( 1,2,3 [...] )
      GROUP BY v.photo_id, v.user_id 
      ) v
WHERE p.user_id = v.user_id AND 
      p.photo_id = v.photo_id AND
      ph.photo_id = p.photo_id AND
      ph.user_id = p.user_id
ORDER BY p.user_id, p.photo_id

EDIT Using left join

SELECT p.user_id, p.photo_id, p.Photos, ph.date, v.Views
FROM (SELECT v.user_id, v.photo_id, count(v.photo_id) as 'Views'
      FROM  userviews v
      WHERE v.photo_id in ( 1,2,3 [...] )
      GROUP BY v.photo_id, v.user_id 
      ) v left join (SELECT p.user_id, p.photo_id, count(p.photo_id) as 'Photos'
      FROM photos p
      WHERE p.photo_id in ( 1,2,3 [...] )
      GROUP BY p.photo_id, p.user_id ) p     
ON p.user_id = v.user_id AND p.photo_id = v.photo_id
      left join photos ph 
On ph.photo_id = p.photo_id AND
      ph.user_id = p.user_id
ORDER BY p.user_id, p.photo_id

When you do not GROUP BY or aggregate all the items in the SELECT list, MySQL chooses the values for the other columns and you might get unexpected results. ( REFER ) that was the reason you were getting wrong results