Question

I am representing itemsets in SQL (SQLite, if relevant). My tables look like this:

ITEMS table:

| ItemId | Name    |
| 1      | Ginseng |
| 2      | Honey   |
| 3      | Garlic  |

ITEMSETS:

| ItemSetId | Name                |
| ...       | ...                 |
| 7         | GinsengHoney        |
| 8         | HoneyGarlicGinseng  |
| 9         | Garlic              |

ITEMSETS2ITEMS

| ItemsetId | ItemId |
| ...       | ....   |
| 7         | 1      |
| 7         | 2      |
| 8         | 2      |
| 8         | 1      |
| 8         | 3      |

As you can see, an Itemset may contain several Items, and this relationship is detailed in the Itemset2Items table.

How can I check whether a new itemset is already in the table, and if so, find its ID?

For instance, I want to check whether “Ginseng, Garlic, Honey” is an existing itemset. The desired answer would be “Yes”, because there exists a single ItemsetId which contains exactly these three IDs. Note that the set is unordered: a query for “Honey, Garlic, Ginseng” should behave identically.

How can I do this?

Answer 1

I would recommend that you start by placing the item sets that you want to check into a table, with one row per item.

The question is now about the overlap of this “proposed” item set to other itemsets. The following query provides the answer:

select itemsetid,
from (select coalesce(ps.itemid, is2i.itemid) as itemid, is2i.itemsetid,
             max(case when ps.itemid is not null then 1 else 0 end) as inProposed,
             max(case when is2i.itemid is not null then 1 else 0 end) as inItemset
      from ProposedSet ps full outer join
           ItemSets2items is2i
           on ps.itemid = is2i.itemid
      group by coalesce(ps.itemid, is2i.itemid), is2i.itemsetid
     ) t
group by itemsetid
having min(inProposed) = 1 and min(inItemSet) = 1

This joins all the proposed items with all the itemsets. It then groups by the items in each item set, giving a flag as to whether the item is in the set. Finally, it checks that all items in an item set are in both.