Question

I am trying to figure out how to use multiple left outer joins to calculate average scores and number of cards. I have the following schema and test data. Each deck has 0 or more scores and 0 or more cards. I need to calculate an average score and card count for each deck. I’m using mysql for convenience, I eventually want this to run on sqlite on an Android phone.

mysql> select * from deck;
+----+-------+
| id | name  |
+----+-------+
|  1 | one   | 
|  2 | two   | 
|  3 | three | 
+----+-------+

mysql> select * from score;
+---------+-------+---------------------+--------+
| scoreId | value | date                | deckId |
+---------+-------+---------------------+--------+
|       1 |  6.58 | 2009-10-05 20:54:52 |      1 | 
|       2 |     7 | 2009-10-05 20:54:58 |      1 | 
|       3 |  4.67 | 2009-10-05 20:55:04 |      1 | 
|       4 |     7 | 2009-10-05 20:57:38 |      2 | 
|       5 |     7 | 2009-10-05 20:57:41 |      2 | 
+---------+-------+---------------------+--------+

mysql> select * from card;
+--------+-------+------+--------+
| cardId | front | back | deckId |
+--------+-------+------+--------+
|      1 | fron  | back |      2 | 
|      2 | fron  | back |      1 | 
|      3 | f1    | b2   |      1 | 
+--------+-------+------+--------+

I run the following query…

mysql> select deck.name, sum(score.value)/count(score.value) "Ave", 
    ->   count(card.front) "Count" 
    -> from deck 
    -> left outer join score on deck.id=score.deckId 
    -> left outer join card on deck.id=card.deckId
    -> group by deck.id;

+-------+-----------------+-------+
| name  | Ave             | Count |
+-------+-----------------+-------+
| one   | 6.0833333333333 |     6 | 
| two   |               7 |     2 | 
| three |            NULL |     0 | 
+-------+-----------------+-------+

… and I get the right answer for the average, but the wrong answer for the number of cards. Can someone tell me what I am doing wrong before I pull my hair out?

Thanks!

John

Answer 1

It’s running what you’re asking–it’s joining card 2 and 3 to scores 1, 2, and 3–creating a count of 6 (2 * 3). In card 1’s case, it joins to scores 4 and 5, creating a count of 2 (1 * 2).

If you just want a count of cards, like you’re currently doing, COUNT(Distinct Card.CardId).