问题:

I have a database that contains two tables:

entries
tags

The entries table contains posts that each have one or more tags. The problem is, each post can have any number of tags. In other words, I can't have a 'tag1', 'tag2', etc. column and do a LEFT JOIN.

How should I set up entries so that each post can have any number of tags?

回答1:

You need a mapping table.

Create a table called entries_tags that contains two columns: entry_id and tag_id, with a multi-key on both entries.

This is called a many-to-many relationship.

回答2:

You can also do it the SO-way, where in addition to having a junction/mapping/intersection table, you have a list of tags on the entry that's taggable:

entries table:
post_id: 3539744, .... tags: sql, database, database-design, tags, data-modeling

If you can't take the performance hit of using a junction table when pulling the associated tags for an entry. Of course extra care has to be taken here because you are dealing with denormalized data.