How to deal with tags in a normalized database?

Question

I am trying to normalize a database I am building. One part of it is that I have many different tests that each can have many different tags. How would you deal with this?

Would you have a table with your tags and then have a limit on the number of tags per test?

I am new to databases let alone the whole idea of normalizing so forgive me if this is a very simplistic question.

Answer 1

Usually, this'd be dealt with using two tables. A tags table, with each record consisting of a tag_id and tag_name (and potentially other metadata you want to include). Tags would then be associated with their tests via another table, test_tags , in which test_id and tag_id would be stored.

If you want to limit the number of tags per test, you can easily run a SELECT COUNT(tag_id) FROM test_tags WHERE test_id=# query to figure out how many they've already added to it.

Answer 2

Usually the way you would do this would be to have one table tags , like:

INT id PRIMARY KEY
VARCHAR(255) name

Then for each object you allow to be tagged, you need a bridge table. So if you have another table tests with its own id column, you would have another table, tests_tags :

INT test_id PRIMARY KEY
INT tag_id PRIMARY KEY

You can then enumerate all tests with a tag by getting the test ids where tag_id is the tag id you are interested in; likewise you can enumerate all tags on a test by getting the tag ids where test_id is the test id you are interested in.

This is the standard way to model a many-to-many relationship.