Database design for apps using “hashtags”

Question

database design question here.

Say we had a webapp or something that uses hashtags for 20-40 word notes. What is the best way to store a user's hashtags.

For instance, if a user entered. "I like to have #lunch at #sizzler" we would store the sentence as text and we could store the hashtags as JSON, a comma separated list or some other mechanism.

Its also worth pointing out that the tags need to be searchable, such as how many people have been hash tagging lunch, etc.

Advise on the matter would be great, I always get a bit stumped when it comes to storing variable sized inputs in mysql. There can be an infinite number of hashtags per note, what is the best way to store them?

Answer 1

I would advise going with a typical many-to-many-relationship between messages and tags.

That would mean you need 3 tables.

• One table for the messages themselves (minimal requirement: columns ID, UserID and message-content)
• One table for the tags (minimal requirement here is: columns ID and tag-name)
• And one last table tagMessagesRelations to make the connections between messages and tags (via foreign keys messageID and tagID)

That way you do not store a tag multiple times but only create a new relation to a message (if that tag already exists in the tag-table of course).

That then enables you to

• easily count how many tags there are (SELECT COUNT(*) FROM tags)
• you will only save each tag once and search for tags can be easily indexed
• or count how many times a certain tag was used per user (for example:

SELECT COUNT(*) FROM tags INNER JOIN tagMessagesRelations ON tags.ID = tagMessagesRelations.tagID INNER JOIN messages ON tagMessagesRelations.messageID = messages.ID GROUP BY messages.UserID