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.