Hierarchical tagging in SQL

Question

I have a PHP web application which uses a MySQL database for object tagging, in which I\'ve used the tag structure accepted as the answer to this SO question.

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Answer 1

I implemented it using two columns. I simplify it here a little, because I had to keep the tag name in a separate field/table because I had to localize it for different languages:

• tag
• path

Look at these rows for example:

tag            path
---            ----
database       database/
mysql          database/mysql/
mysql4         database/mysql/mysql4/
mysql4-1       database/mysql/mysql4-1/
oracle         database/oracle/
sqlserver      database/sqlserver/
sqlserver2005  database/sqlserver/sqlserver2005/
sqlserver2005  database/sqlserver/sqlserver2008/

etc.

Using the like operator on the path field you can easily get all needed tag rows:

SELECT * FROM tags WHERE path LIKE 'database/%'

There are some implementation details like when you move a node in the hierarchy you have to change all children too etc., but it's not hard.

Also make sure that the length of your path is long enough - in my case I used not the tag name for the path, but another field to make sure that I don't get too long paths.