I have two tables, first the table Product:
id|category_id
--+-----------
1 | 12345
2 | 12345
3 | 12465
And then a table activity:
id|prod_id|activity_type |description
--+-------+----------------+-----------
1 | 1 | Initialization | blah
2 | 1 | Finalization | foo
3 | 2 | Initialization | blah again
4 | 2 | Duplication | bar
5 | 2 | Finalization | foobar
6 | 3 | Initialization | blob
7 | 3 | Migration | A to B
8 | 3 | Migration | B to C
9 | 3 | Finalization | fuh
Now I want to retrieve for each type of activity the number of product having at least one of this kind of activity, and also the list of product category. The categories will be repeated in the list for each product of this category. For now I'm using the following query:
SELECT a.activity_type as Activity, COUNT(DISTINCT p.id) as Products,
CONVERT(GROUP_CONCAT(p.category SEPARATOR ', ') USING utf8) AS Categories
FROM mydb.product p, mydb.activity a
WHERE p.id = a.prod_id
AND a.activity_type <> '' // To not count activities which haven't been correctly initialized
GROUP BY Categories
ORDER BY Products
Now what I await for result is:
Activity | Products | Categories
---------------+----------+--------------------
Initialization | 3 | 12345, 12345, 12465
Finalization | 3 | 12345, 12345, 12465
Duplication | 1 | 12345
Migration | 1 | 12465
But with this query I get the value '12465, 12465'
for Migration. I could I get that a category appears on the list, only for each different product ids, but not for each activity of one type?
Best Answer
First group by both
activity_type
andprod_id
and then another group byactivity_type
:Tested in SQL-Fiddle (thank you @Mr.Radical)
You could also safely replace
COUNT(DISTINCT p.id)
withCOUNT(*)
in the above, as for every activity type, there are only distinct product IDs (this is taken care in the internal group by).