Sql – How to design a database schema to support tagging with categories


I am trying to so something like Database Design for Tagging, except each of my tags are grouped into categories.

For example, let's say I have a database about vehicles. Let's say we actually don't know very much about vehicles, so we can't specify the columns all vehicles will have. Therefore we shall "tag" vehicles with information.

1. manufacture: Mercedes
   model: SLK32 AMG
   convertible: hardtop

2. manufacture: Ford
   model: GT90
   production phase: prototype

3. manufacture: Mazda
   model: MX-5
   convertible: softtop

Now as you can see all cars are tagged with their manufacture and model, but the other categories don't all match. Note that a car can only have one of each category. IE. A car can only have one manufacturer.

I want to design a database to support a search for all Mercedes, or to be able to list all manufactures.

My current design is something like this:

  int vid
  String vin

  int vid
  int tid

  int tid
  String tag
  int cid

  int cid
  String category

I have all the right primary and foreign keys in place, except I can't handle the case where each car can only have one manufacturer. Or can I?

Can I add a foreign key constraint to the composite primary key in vehicleTags? IE. Could I add a constraint such that the composite primary key (vid, tid) can only be added to vehicleTags only if there isn't already a row in vehicleTags such that for the same vid, there isn't already a tid in the with the same cid?

My guess is no. I think the solution to this problem is add a cid column to vehicleTags, and make the new composite primary key (vid, cid). It would look like:

  int vid
  int cid
  int tid

This would prevent a car from having two manufacturers, but now I have duplicated the information that tid is in cid.

What should my schema be?

Tom noticed this problem in my database schema in my previous question, How do you do many to many table outer joins?

I know that in the example manufacture should really be a column in the vehicle table, but let's say you can't do that. The example is just an example.

Best Solution

This is yet another variation on the Entity-Attribute-Value design.

A more recognizable EAV table looks like the following:

  vid        INTEGER,
  attr_name  VARCHAR(20),
  attr_value VARCHAR(100),
  PRIMARY KEY (vid, attr_name),
  FOREIGN KEY (vid) REFERENCES vehicles (vid)

Some people force attr_name to reference a lookup table of predefined attribute names, to limit the chaos.

What you've done is simply spread an EAV table over three tables, but without improving the order of your metadata:

CREATE TABLE vehicleTag (
  vid         INTEGER,
  cid         INTEGER,
  tid         INTEGER,
  PRIMARY KEY (vid, cid),
  FOREIGN KEY (vid) REFERENCES vehicles(vid),
  FOREIGN KEY (cid) REFERENCES categories(cid),
  FOREIGN KEY (tid) REFERENCES tags(tid)

CREATE TABLE categories (
  category   VARCHAR(20) -- "attr_name"

  tag        VARCHAR(100) -- "attr_value"

If you're going to use the EAV design, you only need the vehicleTags and categories tables.

CREATE TABLE vehicleTag (
  vid         INTEGER,
  cid         INTEGER,     -- reference to "attr_name" lookup table
  tag         VARCHAR(100, -- "attr_value"
  PRIMARY KEY (vid, cid),
  FOREIGN KEY (vid) REFERENCES vehicles(vid),
  FOREIGN KEY (cid) REFERENCES categories(cid)

But keep in mind that you're mixing data with metadata. You lose the ability to apply certain constraints to your data model.

  • How can you make one of the categories mandatory (a conventional column uses a NOT NULL constraint)?
  • How can you use SQL data types to validate some of your tag values? You can't, because you're using a long string for every tag value. Is this string long enough for every tag you'll need in the future? You can't tell.
  • How can you constrain some of your tags to a set of permitted values (a conventional table uses a foreign key to a lookup table)? This is your "softtop" vs. "soft top" example. But you can't make a constraint on the tag column because that constraint would apply to all other tag values for other categories. You'd effectively restrict engine size and paint color to "soft top" as well.

SQL databases don't work well with this model. It's extremely difficult to get right, and querying it becomes very complex. If you do continue to use SQL, you will be better off modeling the tables conventionally, with one column per attribute. If you have need to have "subtypes" then define a subordinate table per subtype (Class-Table Inheritance), or else use Single-Table Inheritance. If you have an unlimited variation in the attributes per entity, then use Serialized LOB.

Another technology that is designed for these kinds of fluid, non-relational data models is a Semantic Database, storing data in RDF and queried with SPARQL. One free solution is RDF4J (formerly Sesame).