MySQL 左外连接故障

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 (eg 7am), and some hours only have transactions for a single pricepoint (eg 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.

Answer 2

Someone probably has a better solution than this, but I would use a UNION to simplify things:

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

UNION

SELECT hour,0 AS pricepoint,0 AS counter FROM inHour WHERE hour NOT IN (SELECT hour(Stamp) FROM Transactions)