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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.

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    A term can have multiple parents. This is easier represented in the taxonomy_term_hierachy strucutre – J. Reynolds Feb 12 '15 at 16:22
  • @Reynolds, that would make sense, but tid is unqiue (primary key) and the parent field is an integer. Can you explain this? – iStryker Feb 12 '15 at 16:28
  • @iStryker Not quite - it's a compound key across tid + parent, not just on tid – Clive Feb 12 '15 at 16:29
  • My mistake, I did not see the compound key, thought it was just on tid. – iStryker Feb 12 '15 at 16:35
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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.

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