我有两个表，首先是表 Product：
id|category_id
--+-----------
1 | 12345
2 | 12345
3 | 12465
然后是表活动：
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 | …