Recommended SQL database design for tags or tagging

Question

I ve heard of a few ways to implement tagging  using a mapping table between TagID and ItemID  makes sense to me  but does it scale    adding a fixed number of possible TagID columns to ItemID  seems like a bad idea   Keeping tags in a text column that s comma separated  sounds crazy but could work   I ve even heard someone recommend a sparse matrix  but then how do the tag names grow gracefully   Am I missing a best practice for tags

User · Answer

Normally I would agree with Yaakov Ellis but in this special case there is another viable solution:

Use two tables:

Table: Item
Columns: ItemID, Title, Content
Indexes: ItemID

Table: Tag
Columns: ItemID, Title
Indexes: ItemId, Title

This has some major advantages:

First it makes development much simpler: in the three-table solution for insert and update of item you have to lookup the Tag table to see if there are already entries. Then you have to join them with new ones. This is no trivial task.

Then it makes queries simpler (and perhaps faster). There are three major database queries which you will do: Output all Tags for one Item, draw a Tag-Cloud and select all items for one Tag Title.

All Tags for one Item:

3-Table:

SELECT Tag.Title 
  FROM Tag 
  JOIN ItemTag ON Tag.TagID = ItemTag.TagID
 WHERE ItemTag.ItemID = :id

2-Table:

SELECT Tag.Title
FROM Tag
WHERE Tag.ItemID = :id

Tag-Cloud:

3-Table:

SELECT Tag.Title, count(*)
  FROM Tag
  JOIN ItemTag ON Tag.TagID = ItemTag.TagID
 GROUP BY Tag.Title

2-Table:

SELECT Tag.Title, count(*)
  FROM Tag
 GROUP BY Tag.Title

Items for one Tag:

3-Table:

SELECT Item.*
  FROM Item
  JOIN ItemTag ON Item.ItemID = ItemTag.ItemID
  JOIN Tag ON ItemTag.TagID = Tag.TagID
 WHERE Tag.Title = :title

2-Table:

SELECT Item.*
  FROM Item
  JOIN Tag ON Item.ItemID = Tag.ItemID
 WHERE Tag.Title = :title

But there are some drawbacks, too: It could take more space in the database (which could lead to more disk operations which is slower) and it's not normalized which could lead to inconsistencies.

The size argument is not that strong because the very nature of tags is that they are normally pretty small so the size increase is not a large one. One could argue that the query for the tag title is much faster in a small table which contains each tag only once and this certainly is true. But taking in regard the savings for not having to join and the fact that you can build a good index on them could easily compensate for this. This of course depends heavily on the size of the database you are using.

The inconsistency argument is a little moot too. Tags are free text fields and there is no expected operation like 'rename all tags "foo" to "bar"'.

So tldr: I would go for the two-table solution. (In fact I'm going to. I found this article to see if there are valid arguments against it.)

User · Answer

Three tables  one for storing all items  one for all tags  and one for the relation between the two   properly indexed  with foreign keys set running on a proper database  should work well and scale properly    Table  Item Columns  ItemID  Title  Content  Table  Tag Columns  TagID  Title  Table  ItemTag Columns  ItemID  TagID

User · Answer

Use a single formatted text column 1  for storing the tags and use a capable full text search engine to index this  Else you will run into scaling problems when trying to implement boolean queries   If you need details about the tags you have  you can either keep track of it in a incrementally maintained table or run a batch job to extract the information    1  Some RDBMS even provide a native array type which might be even better suited for storage by not needing a parsing step  but might cause problems with the full text search

User · Answer

If you are using a database that supports map-reduce  like couchdb  storing tags in a plain text field or list field is indeed the best way   Example   tagcloud      map  function doc        for tag in doc tags          emit doc tags tag  1               reduce  function keys values       return values length         Running this with group true will group the results by tag name  and even return a count of the number of times that tag was encountered   It s very similar to counting the occurrences of a word in text

User · Answer

I would suggest following design    Item Table   Itemid  taglist1  taglist2 this will be fast and make easy saving and retrieving the data at item level    In parallel build another table   Tags  tag     do not make tag unique identifier and if you run out of space in 2nd column which contains lets say 100 items create another row    Now while searching for items for a tag it will be super fast

User · Answer

I ve always kept the tags in a separate table and then had a mapping table   Of course I ve never done anything on a really large scale either   Having a  tags  table and a map table makes it pretty trivial to generate tag clouds  amp  such since you can easily put together SQL to get a list of tags with counts of how often each tag is used

[sql] Recommended SQL database design for tags or tagging

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