MySQL left outer join trouble

Question

Here is a query that groups transactions by pricepoint on an hourly basis:

SELECT hour(Stamp) AS hour, PointID AS pricepoint, count(1) AS counter
FROM Transactions
GROUP BY 1,2;

Sample output:

+------+------------+---------+
| hour | pricepoint | counter |
+------+------------+---------+
|    0 |         19 |       5 |
|    0 |         20 |      14 |
|    1 |         19 |       3 |
|    1 |         20 |      12 |
|    2 |         19 |       2 |
|    2 |         20 |       8 |
|    3 |         19 |       2 |
|    3 |         20 |       4 |
|    4 |         19 |       1 |
|    4 |         20 |       1 |
|    5 |         19 |       4 |
|    5 |         20 |       1 |
|    6 |         20 |       2 |
|    8 |         19 |       1 |
|    8 |         20 |       4 |
|    9 |         19 |       2 |
|    9 |         20 |       5 |
|   10 |         19 |       6 |
|   10 |         20 |       1 |
|   11 |         19 |      10 |
|   11 |         20 |       2 |
|   12 |         19 |      10 |
|   12 |         20 |       3 |
|   13 |         19 |      10 |
|   13 |         20 |      10 |
|   14 |         19 |       8 |
|   14 |         20 |       3 |
|   15 |         19 |       6 |
|   15 |         20 |       8 |
|   16 |         19 |      11 |
|   16 |         20 |      10 |
|   17 |         19 |       7 |
|   17 |         20 |      17 |
|   18 |         19 |       7 |
|   18 |         20 |       9 |
|   19 |         19 |      10 |
|   19 |         20 |      12 |
|   20 |         19 |      17 |
|   20 |         20 |      11 |
|   21 |         19 |      12 |
|   21 |         20 |      29 |
|   22 |         19 |       6 |
|   22 |         20 |      21 |
|   23 |         19 |       9 |
|   23 |         20 |      23 |
+------+------------+---------+

As you can see, some hours have no transactions (e.g 7am), and some hours only have transactions for a single pricepoint (e.g. 6am, only pricepoint 20 but no transactions for pricepoint 19).

I would like to display the results set with "0" when there are no transactions, rather than just not being there as is the case now.

Trying to work with a LEFT OUTER JOIN there. The inHour table contains values 0..23

SELECT H.hour, PointID AS Pricepoint, COALESCE(T.counter, 0) AS Count
FROM inHour H
LEFT OUTER JOIN
(
 SELECT hour(Stamp) AS Hour, PointID, count(1) AS counter
 FROM Transactions
 GROUP BY 1,2
 ) T
ON T.Hour = H.hour;

This produces the following output (truncated for brevity):

|    5 |         19 |     4 |
|    5 |         20 |     1 |
|    6 |         20 |     2 |
|    7 |       NULL |     0 |
|    8 |         19 |     1 |
|    8 |         20 |     4 |

What I would like in fact would be:

|    5 |         19 |     4 |
|    5 |         20 |     1 |
|    6 |         19 |     0 |
|    6 |         20 |     2 |
|    7 |         19 |     0 |
|    7 |         20 |     0 |
|    8 |         19 |     1 |
|    8 |         20 |     4 |

In my desired output, the value "0" is put next to pricepoints that had no transactions during a given hour.

Your suggestions would be welcome! Thanks.

Answer 1

SELECT h.Hour, p.Pricepoint, COUNT(t.*) AS Count
FROM inHour h,
(SELECT DISTINCT PointId AS Pricepoint FROM Transactions) p
LEFT OUTER JOIN Transactions t
ON h.Hour = hour(t.Stamp) AND p.Pricepoint = t.PointID
GROUP BY h.Hour, p.Pricepoint
ORDER BY h.Hour, p.Pricepoint

I don't have time at the moment to try this, so let me know if it doesn't work and I'll try to adjust.