Arsthbt

I am a PHP developer and recently I am trying to build a platform which needs lots of categories/filters/tags some in hierarchy some not.

So the question is...

I am able to manage terms in these tables
terms, terms_metadata and terms_relationships, but there is another table in WordPress - terms_taxanomymy.

What is the significance of defining another table for just storing "parent", "type", "count" and "description" when you can do that in terms table itself.

Well, it uses 3 tables. The fourth one (wp_termmeta) is just a way of allowing you to store some meta values for terms (icons, additional descriptions and so on).

Citing from Codex:

• 
  wp_terms - The categories for both posts and links and the tags for posts are found within the wp_terms table.


• 
  wp_termmeta - Each term features information called the meta data and it is stored in wp_termmeta.


• 
  wp_term_relationships - Posts are associated with categories and tags from the wp_terms table and this association is maintained in
  the wp_term_relationships table. The association of links to their
  respective categories are also kept in this table.


• 
  wp_term_taxonomy - This table describes the taxonomy (category, link, or tag) for the entries in the wp_terms table.

So the only part that may be confusing is that there are separate tables for terms and term_taxonomy... And it can be confusing, because it has lost its meaning nowadays.

But before WP 4.2.2 (I guess) the approach to terms was a little bit different - terms were unique across wp_terms table.

So if you had “books” as category and “books” as tag, then there was only one record with “books” in wp_terms and obviously 2 records in wp_term_taxonomy.

But when the wp_termmeta table was introduced it would make it confusing - so such terms were split. And today, for the example above, you’ll have two separate rows in wp_terms table.