>For good and sufficient reasons I wish to insure that a primary key of table 1 is not a primary key of table 2. <<
> The following does not work:
ALTER TABLE IS3.AUCTION_SUP ER_CATEGORIES
ADD CONSTRAINT code_not_fk
check(code not in
(select code from IS3.AUCTION_CAT EGORIES
where auction_id=auct ion_id));
<<
What **kind of code** did you mean? You need ISO-11179 names here.
To be is to be something in particular. And shouldn't you qualify
auction_id? Then let's follow Mother Celko's formattign rules about
key words in uppercase.
ALTER TABLE IS3.auction_sup er_categories
ADD CONSTRAINT vague_code_not_ fk
CHECK(vague_cod e NOT IN
(SELECT vague_code
FROM IS3.auction_cat egories AS A
WHERE auction_super_c ategories.aucti on_id =
A.auction_id));
It still has problems, but at least it looks nice now :)
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_t ype 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_t ype = '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_t ype = '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_t ype 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_t ype = '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_t ype = '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_t ype 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.
Another approach is to design a Dewey Decimal hierarchical
auction_categor ies and put restrictions on it based on ranges.
