Question

Consider I have a table with notes which could be associated with zero or more tags, how would I decide to do

create table notes (
  id int , -- primary key
  -- other fields
);
create table tagmapping (
 noteid int, -- refers notes.id
 tagid int, -- refers tags.id
 );
create table tags (
  int id, -- primary key
  tagname varchar(255)
);

vs storing the same tag potentially several times as in

create table notes (
  int id, -- primary key
  -- other fields

);

create table bar (
  id id, -- primary key
  tag varchar(255),
  -- other fields
  noteid int -- refers to notes.id, (not unique)
);

What mess/advantages would I get myself into going for the last approach ?

Answer 1

The first way is the standard way of implementing a many-to-many relationship. You should do this because this is what will surprise people the least.

A couple of issues with the second method:

• You will have to duplicate the tag names, which introduces redundancy. It could be a waste of disk space if the tag names are long.
• If you ever want to rename a tag, you have to update the entire table instead of just one field.