CREATE TABLE

Company Table

cname	country	no_employees	for_profit
Canon	Japan	50000	Y
Hitachi	Japan	30000	Y

CREATE TABLE Company (
  cname VARCHAR(30),
  country VARCHAR(20),
  no_employees INT,
  for_profit BOOLEAN
)

or:

CREATE TABLE Company (
  cname VARCHAR(30) PRIMARY KEY,
  country VARCHAR(20),
  no_employees INT,
  for_profit BOOLEAN
)

Product(name, category)

CREATE TABLE Product (
  name CHAR(30) PRIMARY KEY,
  category VARCHAR(20)
)

or:

CREATE TABLE Product (  
    name CHAR(30),  
    category VARCHAR(20),  
    PRIMARY KEY (name)
)

Keys with multiple attributes

Product(name, category, price)

CREATE TABLE Product (  
    name CHAR(30),  
    category VARCHAR(20),  
    price INT,  
    PRIMARY KEY (name, category)  
)

Name	Category	Price
Gizmo	Gadget	10
Camera	Photo	20
Gizmo	Photo	30
Gizmo	Gadget	40

Other keys

CREATE TABLE Product (  
    productID CHAR(10),  
    name CHAR(30),  
    category VARCHAR(20),  
    price INT,  
    PRIMARY KEY (productID),  
    UNIQUE (name, category))

There is at most one PRIMARY KEY;
there can be many UNIQUE

Feature	PRIMARY KEY	UNIQUE
Ensures uniqueness	✅ Yes	✅ Yes
Allows NULL values	❌ No (NULL not allowed)	✅ Yes (unless you say otherwise)
Number per table	❗Only one allowed	✅ You can have multiple UNIQUE constraints
Main identity	✅ Used to identify each row	❌ Just used to enforce rules, not row identity

Foreign key constraints

CREATE TABLE Purchase (  
    prodName CHAR(30) REFERENCES Product(name),  
    date DATETIME)

prodName is a foreign key to Product(name)
name must be a key in Product

Example:

CREATE TABLE Purchase (  
    prodName CHAR(30),  
    category VARCHAR(20),  
    date DATETIME,  
    FOREIGN KEY (prodName, category)  
    REFERENCES Product(name, category))

(name, category) must be a KEY in Product