MYSQL Count not correct - mysql

Question

SELECT
b.cID,
b.ID,
Count(r.userRead) AS readCount,
COUNT(DISTINCT r.userID) AS UserCount,
Count(c.userDownload) AS downloadCount,
COUNT(DISTINCT c.userID) AS userDownloadCount
FROM
book AS b
INNER JOIN book_event AS r ON r.bookID=s.ID AND r.bookRead = 1
INNER JOIN book_event as c ON c.bookID=s.ID AND c.bookDownload = 1
WHERE
b.cID = 1011
GROUP BY
b.ID
ORDER BY
b.ID DESC
this SQL query output (count's problem)
+-----------+-----+-----------+-----------------+--------------+-------------------+
| cID | ID | readCount | UserCount | downloadCount| userDownloadCount |
+-----------+-----+-----------+-----------------+--------------+-------------------+
| 1011 | 278 | 3168 | 67 | 3168 | 19 |
| 1011 | 272 | 9918 | 122 | 9918 | 41 |
| 1011 | 241 | 31694 | 99 | 31694 | 38 |
+-----------+-----+-----------+-----------------+--------------+-------------------+
3 rows in set
real value
+-----------+-----+-----------+-----------------+--------------+-------------------+
| cID | ID | readCount | UserCount | downloadCount| userDownloadCount |
+-----------+-----+-----------+-----------------+--------------+-------------------+
| 1011 | 278 | 133 | 67 | 24 | 19 |
| 1011 | 272 | 174 | 122 | 57 | 41 |
| 1011 | 241 | 299 | 99 | 106 | 38 |
+-----------+-----+-----------+-----------------+--------------+-------------------+
book_event (table)
+-----+--------+----------+--------------+
| ID | userID | userRead | userDownload |
+-----+--------+----------+--------------+
| 278 | 5169 | 1 | 0 |
| 278 | 5169 | 0 | 1 |
| ... | .... | . | . |
| 278 | 5628 | 1 | 0 |
| 278 | 5162 | 1 | 0 |
+-----+--------+----------+--------------+
I need to get the count grouped on two columns. readCount and downloadCount columuns is not correct but UserCount, userDownloadCount columuns value is correct.
how can i fix this problem?

Answer 1

This is because you have multiple read and download events on the same book, so your query is generating a cross product of events.
A good way to fix this is to aggregate the pieces of information separately. However, your query offers an easier solution. Just join to the book_event table once and then count the different events.
SELECT b.cID, b.ID,
sum(be.bookRead) as readCount,
count(distinct case when be.bookRead = 1 then be.userId end) as UserCount,
sum(be.userDownload) as downloadCount,
count(distinct case when be.userDownload = 1 then be.userId end) as userDownloadCount
FROM book AS b INNER JOIN
book_event be
on be.bookID = s.ID
WHERE b.cID = 1011
GROUP BY b.ID, b.cid
ORDER BY b.ID DESC
I added b.cid to the group by clause. It is good form to include all non-aggregated values in the SELECT clause in the GROUP BY. Other databases enforce this, and the rule is standard SQL.

Answer 2

I think your query is technically wrong (the group by syntax), but it is unrelated to your problem and works on MySQL.
The most common cause of count issues is not correctly accounting for nulls - but I don't think this is your problem either.
try this
SELECT
b.cID,
b.ID,
sum(distinct coalesce(r.userRead,0)) AS readCount,
count(DISTINCT r.userID) AS UserCount,
sum(distinct coalesce(c.userDownload,0)) AS downloadCount,
count(DISTINCT c.userID) AS userDownloadCount
FROM
book AS b
left JOIN book_event AS r ON r.bookID=s.ID AND r.bookRead = 1
left JOIN book_event as c ON c.bookID=s.ID AND c.bookDownload = 1
WHERE
b.cID = 1011
GROUP BY
b.cID, b.ID
ORDER BY
b.ID DESC