SQL：SUM不能与COUNT一起使用，但可以单独使用

Question

I'm trying to execute the following SQL query in SQL Server 2008

String query = "SELECT SUM(r.rate),COUNT(q.best_answer_id) " +
                            "FROM questions_rating r,questions q " +
                            "WHERE r.question_id IN (SELECT question_id FROM questions WHERE user_id = 1) "+
                            "AND q.best_answer_id IN (SELECT answer_id FROM answers WHERE user_id = 1)";

These are the Tables

Questions_Rating

id---------question_id-----------user_id--------rate
1               1                    1            1
2               1                    2            1
3               1                    3           -1
4               2                    1           -1

The rate can only be either 1 or -1.

Questions

question_id------question-------user_id-------best_answer_id
1                   lala            1             3
2                   lala            2             5

Answers

answer_id---------answer--------user_id------question_id
1                   lala            4             1
2                   kaka            5             1
3                   dada            6             1
4                   fafa            7             2
5                   tata            8             2

The query is returning these results

SUM------COUNT
NULL       0

While it should return...

SUM------COUNT
1          0

Note that the following query returns the right result (1)

String query = "SELECT SUM(r.rate) " +
                            "FROM questions_rating r " +
                            "WHERE r.question_id IN (SELECT question_id FROM questions WHERE user_id = 1) ";

Answer 1

I think you should consider rewriting your query to use JOINS instead of your subqueries:

SELECT 
  SUM(r.rate) SumOfRate, 
  COUNT(a.answer_id) CountOfBest
FROM questions_rating r
INNER JOIN questions q
  ON r.question_id = q.question_id
  AND r.user_id = q.user_id
LEFT JOIN answers a
  ON q.best_answer_id = a.answer_id
  AND a.user_id = 1
WHERE r.user_id = 1

See SQL Fiddle with Demo

Returns the result:

| SUMOFRATE | COUNTOFBEST |
---------------------------
|         1 |           0 |

Answer 2

When you join the tables you're getting NULL values for r.rate. Try using the ISNULL function to check for NULL values like this:

String query = "SELECT SUM(ISNULL(r.rate,0)),COUNT(ISNULL(q.best_answer_id),0)) " +
                            "FROM questions_rating r,questions q " +
                            "WHERE r.question_id IN (SELECT question_id FROM questions WHERE user_id = 1) "+
                            "AND q.best_answer_id IN (SELECT answer_id FROM answers WHERE user_id = 1)";

Answer 3

AND q.best_answer_id IN (SELECT answer_id FROM answers WHERE user_id = 1)

You have no entries where user_id = 1 in the answers table, so this subquery will return no answer_ids. You have effectively excluded all rows with this filter and your aggregates are properly calculating them as such.

The query that returned results did not have this line as part of the WHERE filter.

Answer 4

The most likely explanation is that your query is not returning any rows.

I suggest you remove the aggregates, and check the results, eg:

SELECT r.rate
     , q.best_answer_id
  FROM questions_rating r
 CROSS
  JOIN questions q
 WHERE r.question_id IN
       (SELECT question_id FROM questions WHERE user_id = 1)
   AND q.best_answer_id IN
       (SELECT answer_id FROM answers WHERE user_id = 1)

I believe you will find that there are no rows returned. In that case, a COUNT() aggregate will reasonably return a 0, and a SUM() aggregate will return a NULL.

It's fairly easy to modify the query to replace the NULL value with a zero, but in this case, I don't believe that is your issue. (It's not at all clear why you would want a Cartesian product of the questions and questions_rating tables.)

I think your first step needs to be getting the rows returned that you need returned, and then work on getting the aggregates applied.