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Re: How to model searchable properties of an entity

From: --CELKO-- <jcelko212_at_earthlink.net>
Date: 13 Aug 2004 18:58:21 -0700
Message-ID: <18c7b3c2.0408131758.65f91710@posting.google.com>


>> Let us say I have Part# and it has very large number of flexible
attributes defined by the user, such as color, width, height, etc... <<

Fire the DBA immediately; users NEVER get to define the database and this bum is not doing his job.

>> Depending on Part number, these attributes may change.<<

No, the values of the attributes will change. If the attributes change, we have a new entity, by definition. Or a bad data model.

>> To make the things worse, they are searchable, meaning show me all
the parts which are blue in color and whose width is less than 10 inches, etc. <<

If the goods are that weird and changable, look at using a Google or EBay document search engine. Put a relational inventory system on the front end tpo track the document side of the house.

Very few businesses work this way, just as very people actually search on oddball random attributes.

>> If I make, name value pair, them my search will has as many self
joins
as I have ands in my query. <<

This design error is called EAV (entity-attribute-value) and it is common enough to have a name -- like "cancer" :)

The classic scenario calls for a root class with all the common attributes and then specialized sub-classes under it. As an example, let's take the class of Vehicles and find an industry standard identifier (VIN), and add two mutually exclusive sub-classes, Sport utility vehicles and sedans ('SUV', 'SED').

CREATE TABLE Vehicles
(vin CHAR(17) NOT NULL PRIMARY KEY,
 vehicle_type CHAR(3) NOT NULL

       CHECK(vehicle_type IN ('SUV', 'SED')),  UNIQUE (vin, vehicle_type),
 ..);

Notice the overlapping candidate keys. I then use a compound candidate key (vin, vehicle_type) and a constraint in each sub-class table to assure that the vehicle_type is locked and agrees with the Vehicles table. Add some DRI actions and you are done:

CREATE TABLE SUV
(vin CHAR(17) NOT NULL PRIMARY KEY,
 vehicle_type CHAR(3) DEFAULT 'SUV' NOT NULL

       CHECK(vehicle_type = 'SUV'),
 UNIQUE (vin, vehicle_type),
 FOREIGN KEY (vin, vehicle_type)
  REFERENCES Vehicles(vin, vehicle_type)   ON UPDATE CASCADE
  ON DELETE CASCADE,
 ..);

CREATE TABLE Sedans
(vin CHAR(17) NOT NULL PRIMARY KEY,
 vehicle_type CHAR(3) DEFAULT 'SED' NOT NULL

       CHECK(vehicle_type = 'SED'),
 UNIQUE (vin, vehicle_type),
 FOREIGN KEY (vin, vehicle_type)
  REFERENCES Vehicles(vin, vehicle_type)   ON UPDATE CASCADE
  ON DELETE CASCADE,
 ..);

I can continue to build a hierarchy like this. For example, if I had a Sedans table that broke down into two-door and four-door sedans, I could a schema like this:  

CREATE TABLE Sedans
(vin CHAR(17) NOT NULL PRIMARY KEY,
 vehicle_type CHAR(3) DEFAULT 'SED' NOT NULL

       CHECK(vehicle_type IN ('2DR', '4DR', 'SED')),  UNIQUE (vin, vehicle_type),
 FOREIGN KEY (vin, vehicle_type)
  REFERENCES Vehicles(vin, vehicle_type)   ON UPDATE CASCADE
  ON DELETE CASCADE,
 ..);

CREATE TABLE TwoDoor
(vin CHAR(17) NOT NULL PRIMARY KEY,
 vehicle_type CHAR(3) DEFAULT '2DR' NOT NULL

       CHECK(vehicle_type = '2DR'),
 UNIQUE (vin, vehicle_type),
 FOREIGN KEY (vin, vehicle_type)
  REFERENCES Sedans(vin, vehicle_type)
  ON UPDATE CASCADE
  ON DELETE CASCADE,
 ..);

CREATE TABLE FourDoor
(vin CHAR(17) NOT NULL PRIMARY KEY,
 vehicle_type CHAR(3) DEFAULT '4DR' NOT NULL

       CHECK(vehicle_type = '4DR'),
 UNIQUE (vin, vehicle_type),
 FOREIGN KEY (vin, vehicle_type)
  REFERENCES Sedans (vin, vehicle_type)
  ON UPDATE CASCADE
  ON DELETE CASCADE,
 ..);

The idea is to build a chain of identifiers and types in a UNIQUE() constraint that go up the tree when you use a REFERENCES constraint. Obviously, you can do variants of this trick to get different class structures.

If an entity doesn't have to be exclusively one subtype, you play with the root of the class hierarchy:

CREATE TABLE Vehicles
(vin CHAR(17) NOT NULL,
 vehicle_type CHAR(3) NOT NULL

       CHECK(vehicle_type IN ('SUV', 'SED')),  PRIMARY KEY (vin, vehicle_type),
 ..);

Now start hiding all this stuff in VIEWs immediately and add an INSTEAD OF trigger to those VIEWs. Received on Fri Aug 13 2004 - 20:58:21 CDT

Original text of this message

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