Question

I need to implement following categorization in SQL:

• group can contain multiple tests or multiple (child) groups
• test could be in multiple groups
• (child) group could be in multiple (parent) groups

… that means 2 x M:N relations. Following tables could store the group trees:

TEST_TABLE
test_id (pk), test_name

GROUP_TABLE
group_id (pk), group_name

TEST_IN_GROUP_TABLE
test_id (fk), group_id (fk)

GROUP_RELATIONS_TABLE
parent_group_id (fk), child_group_id (fk)

But:

• How to list all tests in group? This sounds to me impossibly difficult, since group could contain tests or sub-groups with another tests.
• Or how to modify tables to make such SELECT possible? I could create additional child_count column in GROUP_TABLE. It will be auto-filled by trigger. But even with it it’s hard nut for me.

Answer 1

You use Common Table Expressions to first build up the group_ids of the group sought, then using that single list of groups, join through TEST_IN_GROUP_TABLE to the test table for all tests.

;with cte as (
    select group_id
    from GROUP_TABLE
    where group_id = 1
    union all
    select child_group_id
    from GROUP_RELATIONS_TABLE
    inner join cte on cte.group_id = GROUP_RELATIONS_TABLE.parent_group_id
)
select t.*
from cte
inner join TEST_IN_GROUP_TABLE tg on tg.group_id = cte.group_id
inner join TEST_TABLE t on t.test_id = tg.test_id