Question

I have an sqlite table “log” that looks like this:

ID     p_id     viewer
----------------------
1      1        100
2      1        200
3      1        300
4      3        550
5      3        230
6      5        420
7      2        320
8      2        203
9      9        10
10     9        55

And I want to get the average viewers from each p_id. That’d be

SELECT avg(viewer) FROM log GROUP BY p_id

But I want to treat p_id 1 and 5 as the same, so I’d get the average viewers of p_id 1 and 5 combined. How do I do that? Note that the table is much larger, and I need to treat two p_id’s as the same multiple times. Can I still do a “group by” to achieve this, or are there other ways?

Answer 1

I tried this and it works with mySQL at least.

SELECT avg(viewer) FROM log GROUP BY CASE p_id WHEN 5 THEN 1 ELSE p_id END;

Edit: When using an alias-table named ‘aliases’ with the fields ‘alias_from_id’ and ‘aliased_as’:

SELECT
CASE ISNULL((SELECT aliased_as FROM aliases WHERE (alias_from_id = log.pid))) 
WHEN 1 THEN log.pid ELSE aliased_as END AS the_id,
SUM(value) AS value_sum
FROM log
LEFT JOIN aliases ON (alias_from_id = pid)
GROUP BY
CASE ISNULL((SELECT aliased_as FROM aliases WHERE (alias_from_id = log.pid)))
WHEN 1 THEN log.pid ELSE aliased_as END 

I’ve tested this with mySQL and it works like a charm. It might be possible to simplify this SQL-query a bit but this is the best I can do at the moment 🙂

Edit2: Changed ISNULL to the corresponding SQLite IFNULL function

SELECT
IFNULL((SELECT aliased_as FROM aliases WHERE (alias_from_id = log.pid)), log.pid) AS the_id,
SUM(value) AS value_sum
FROM log
LEFT JOIN aliases ON (alias_from_id = pid)
GROUP BY
IFNULL((SELECT aliased_as FROM aliases WHERE (alias_from_id = log.pid)), log.pid)

I don’t know if SQLite supports the AS keyword, if it doesn’t then just remove that keyword – the functionality should be the same.