Question

I am relatively new to database design. I have a database which has the following design

I have specified the the tables in SQL as follows.

CREATE TABLE Piece
(identifier INT NOT NULL Unique,
 value      INT NOT NULL,
 the date it was acquited DATE,
 the date it was made DATE,
 PRIMARY KEY (identifier));

CREATE TABLE Person
(name VARCHAR(50),
 person_id INT NOT NULL Unique,
 biography VARCHAR (50),
 date of birth DATE,
 date of death DATE,
 PRIMARY KEY (person_id));

CREATE TABLE Jewel
(code INT NOT NULL Unique,
 gem type VARCHAR (50),
 weight INT,
 quality VARCHAR (50),
 color VARCHAR (50),
 description VARCHAR (50),
 PRIMARY KEY (code));

CREATE TABLE Gem
(type VARCHAR (50) NOT NULL,
 hardness INT,
 density  INT,
 FOREIGN KEY (type) references JEWEL(gem type));

CREATE TABLE Ownership
(person VARCHAR (50),
 piece  INT,
 start of ownership DATE,
 end of ownership   DATE,
 FOREIGN KEY (person) references PERSON(person_id),
 FOREIGN KEY (piece) references PIECE(identifier)); 

My question is

1.) How can I specify a primary key to the GEM table as all the 3 attributes are not unique, should I have to create a new attribute like Gem_id, as I would prefer to use the existing attributes and not add a new attribute.

2.) I have used person_id attribute in the person table to make it unique and use it as a primary key, is there another way to create a primary key for the person table without adding the extra attribute and obviously I cant include constraints to the existing attribute and make it UNIQUE

3.) Is all integrity constraints and data types right ? Is my design flawed in any way.

Answer 1

1) Yes you can create a multi column primary key (composite primary key) but it will be unique. Something like this should do it:

CREATE TABLE Gem
(type VARCHAR (50) NOT NULL,
 hardness INT,
 density  INT,
 PRIMARY KEY (type , hardness, density ),
 FOREIGN KEY (type) references JEWEL(gem type));

2) There are other ways to create a primary key for the Person table but I would not recommend it. Person_id is what I would use here.

3) I would make the following changes to your design:

• Make Person in the ownership table an int rather than a VARCHAR. If it’s a foreign key to person_id, then you don’t want it to be a VARCHAR.
• Add an int identifier to the Gem table. GemTypeId or something.
• In the jewel table, replace the gem type column with your new GemTypeId column.

There might be more, but these were the ones that jumped out at me.

It seems like you are trying not to use IDs to represent your data. Is there a reason for this? If you continue down this road you may run into data integrity issues.