Question

It’s been a long time since I designed a database and just want to make sure that I am designing it properly.

I have an Entity which has a name and description.

This Entity can have many subEntities which are of type Entity.

So its like a recursive relationship. Now sure how to design the db properly.

Do I create a second table or what?

Update: One entity can only have a single parent entity or no parents.

Answer 1

Edit: Based on the update to the question, my suggested solution is in bold, below:

First, let’s be clear whether this is really a many-to-many situation because it’s not clear from your description whether that’s true or not.

You describe a tree structure in which an entity can have “nested” entities and, presumably, those nested entities can each have their own nested entities. But this is only a many-to-many question if a single entity can belong, simultaneously to multiple parents.

In either case, you will store all the entities in a single table. If you want to represent a “simple” tree of nested entities without multiple-parentage then you can add a parent_entity_id column to the entities table which points to the entity_id of the parent. Top level entities would have either a flag value (for instance, -1) or a NULL in this column. In this case, you need only one more column and no new tables to represent the relationship.

If it’s really a many-to-many relationship with multiple-parentage then you will create a new table entity_links with columns parent_entity_id and child_entity_id. You manage the relationships by inserting and deleting rows in this table.