Why are there two database tables to handle taxonomy terms structure

Question

There is a taxonomy_term_data table and there is a taxonomy_term_hierarchy table. Why are there 2 tables, instead of them being combined?

taxonomy_term_hierachy strucutre

• description of table: Stores the hierarchical relationship between
• field: tid: 'Primary Key: the taxonomy_term_data.tid of the term'
• field: parent: 'Primary Key: The taxonomy_term_data.tid of the term's parent. 0 indicates no parent.'

taxonomy_term_data structure:

• description of table: Stores term information.
• fields: vid, name, description, format, weight

Question motivation I am the maintainer of the draggableviews module. Currently module is using just 1 table. Going forward, the module's goal is to mimic taxonomy structure. Tried to search the web for an answer but could not find one.

Answer 1

taxonomy_term_data is the base table for the taxonomy entity type. It holds ID, property and bundle data for that entity type.

taxonomy_term_hierarchy holds the parent/child relationship between taxonomy terms (which of course can be multiple in nature).

You can't store the data for anything more than a one-to-one relationship in the same table, hence the split into two.