Re: foreign key constraint to multiple tables

> Is there a way to set the item_type_id column in Table A as a foreign key to
> the other tables depending? Or would there be a better way to design this
> schema?

There is another that that uses candidate keys instead of autonumber keys:

CREATE TABLE Types (
type_name varchar( 100 ) NOT NULL UNIQUE,
type_code varchar( 5 ) NOT NULL
CHECK ( type_code IN ( 'TypeA', 'TypeB', 'TypeC' )),

CONSTRAINT Types_Primary_key
PRIMARY KEY ( type_name, type_code )
);

CREATE TABLE TypeA (
name_A varchar( 100 ) NOT NULL UNIQUE,
type_code varchar( 5 ) NOT NULL DEFAULT 'TypeA'
CHECK ( type_code = 'TypeA'),
attribute_of_A text NOT NULL,

CONSTRAINT Types_Primary_key
PRIMARY KEY ( name_A, type_code )
REFERENCES Types ( type_name, type_code )
);

CREATE TABLE TypeB (
name_B varchar( 100 ) NOT NULL UNIQUE,
type_code varchar( 5 ) NOT NULL DEFAULT 'TypeB'
CHECK ( type_code = 'TypeB'),
attribute_of_B numeric(10,4) NOT NULL,

CONSTRAINT Types_Primary_key
PRIMARY KEY ( name_A, type_code )
REFERENCES Types ( type_name, type_code )
);

CREATE TABLE TypeC (
name_C varchar( 100 ) NOT NULL UNIQUE,
type_code varchar( 5 ) NOT NULL DEFAULT 'TypeC'
CHECK ( type_code = 'TypeC'),
attribute_of_C TimeStamp With Time Zone NOT NULL
DEFAULT CURRENT_TIMESTAMP,

CONSTRAINT Types_Primary_key
PRIMARY KEY ( name_A, type_code )
REFERENCES Types ( type_name, type_code )
);