In the following example - Database for an auto shop business, we have a list of departments, employees, customers and customer cars. We are using foreign keys to create relationships between the various tables.

Live example: SQL fiddle

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Relationships between tables

• Each Department may have 0 or more Employees
• Each Employee may have 0 or 1 Manager
• Each Customer may have 0 or more Cars

---

Departments

Id	Name
1	HR
2	Sales
3	Tech

SQL statements to create the table:

CREATE TABLE Departments (
    Id INT NOT NULL AUTO_INCREMENT,
    Name VARCHAR(25) NOT NULL,
    PRIMARY KEY(Id)
);

INSERT INTO Departments
    ([Id], [Name])
VALUES
    (1, 'HR'),
    (2, 'Sales'),
    (3, 'Tech')
;

---

Employees

Id	FName	LName	PhoneNumber	ManagerId	DepartmentId	Salary	HireDate
1	James	Smith	1234567890	NULL	1	1000	01-01-2002
2	John	Johnson	2468101214	1	1	400	23-03-2005
3	Michael	Williams	1357911131	1	2	600	12-05-2009
4	Johnathon	Smith	1212121212	2	1	500	24-07-2016

SQL statements to create the table:

CREATE TABLE Employees (
    Id INT NOT NULL AUTO_INCREMENT,
    FName VARCHAR(35) NOT NULL,
    LName VARCHAR(35) NOT NULL,
    PhoneNumber VARCHAR(11),
    ManagerId INT,
    DepartmentId INT NOT NULL,
    Salary INT NOT NULL,
    HireDate DATETIME NOT NULL,
    PRIMARY KEY(Id),
    FOREIGN KEY (ManagerId) REFERENCES Employees(Id),
    FOREIGN KEY (DepartmentId) REFERENCES Departments(Id)
);

INSERT INTO Employees
    ([Id], [FName], [LName], [PhoneNumber], [ManagerId], [DepartmentId], [Salary], [HireDate])
VALUES
    (1, 'James', 'Smith', 1234567890, NULL, 1, 1000, '01-01-2002'),
    (2, 'John', 'Johnson', 2468101214, '1', 1, 400, '23-03-2005'),
    (3, 'Michael', 'Williams', 1357911131, '1', 2, 600, '12-05-2009'),
    (4, 'Johnathon', 'Smith', 1212121212, '2', 1, 500, '24-07-2016')
;

---

Customers

Id	FName	LName	Email	PhoneNumber	PreferredContact
1	William	Jones	[email protected]	3347927472	PHONE
2	David	Miller	[email protected]	2137921892	EMAIL
3	Richard	Davis	[email protected]	NULL	EMAIL

SQL statements to create the table:

CREATE TABLE Customers (
    Id INT NOT NULL AUTO_INCREMENT,
    FName VARCHAR(35) NOT NULL,
    LName VARCHAR(35) NOT NULL,
    Email varchar(100) NOT NULL,
    PhoneNumber VARCHAR(11),
    PreferredContact VARCHAR(5) NOT NULL,
    PRIMARY KEY(Id)
);

INSERT INTO Customers
    ([Id], [FName], [LName], [Email], [PhoneNumber], [PreferredContact])
VALUES
    (1, 'William', 'Jones', '[email protected]', '3347927472', 'PHONE'),
    (2, 'David', 'Miller', '[email protected]', '2137921892', 'EMAIL'),
    (3, 'Richard', 'Davis', '[email protected]', NULL, 'EMAIL')
;

---

Cars

Id	CustomerId	EmployeeId	Model	Status	Total Cost
1	1	2	Ford F-150	READY	230
2	1	2	Ford F-150	READY	200
3	2	1	Ford Mustang	WAITING	100
4	3	3	Toyota Prius	WORKING	1254

SQL statements to create the table:

CREATE TABLE Cars (
    Id INT NOT NULL AUTO_INCREMENT,
    CustomerId INT NOT NULL,
    EmployeeId INT NOT NULL,
    Model varchar(50) NOT NULL,
    Status varchar(25) NOT NULL,
    TotalCost INT NOT NULL,
    PRIMARY KEY(Id),
    FOREIGN KEY (CustomerId) REFERENCES Customers(Id),
    FOREIGN KEY (EmployeeId) REFERENCES Employees(Id)
);

INSERT INTO Cars
    ([Id], [CustomerId], [EmployeeId], [Model], [Status], [TotalCost])
VALUES
    ('1', '1', '2', 'Ford F-150', 'READY', '230'),
    ('2', '1', '2', 'Ford F-150', 'READY', '200'),
    ('3', '2', '1', 'Ford Mustang', 'WAITING', '100'),
    ('4', '3', '3', 'Toyota Prius', 'WORKING', '1254')
;

In this example database for a library, we have Authors, Books and BooksAuthors tables.

Live example: SQL fiddle

Authors and Books are known as base tables, since they contain column definition and data for the actual entities in the relational model. BooksAuthors is known as the relationship table, since this table defines the relationship between the Books and Authors table.

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Relationships between tables

• Each author can have 1 or more books
• Each book can have 1 or more authors

---

Authors

(view table)

Id	Name	Country
1	J.D. Salinger	USA
2	F. Scott. Fitzgerald	USA
3	Jane Austen	UK
4	Scott Hanselman	USA
5	Jason N. Gaylord	USA
6	Pranav Rastogi	India
7	Todd Miranda	USA
8	Christian Wenz	USA

SQL to create the table:

CREATE TABLE Authors (
    Id INT NOT NULL AUTO_INCREMENT,
    Name VARCHAR(70) NOT NULL,
    Country VARCHAR(100) NOT NULL,
    PRIMARY KEY(Id)
);

INSERT INTO Authors
    (Name, Country)
VALUES
    ('J.D. Salinger', 'USA'),
    ('F. Scott. Fitzgerald', 'USA'),
    ('Jane Austen', 'UK'),
    ('Scott Hanselman', 'USA'),
    ('Jason N. Gaylord', 'USA'),
    ('Pranav Rastogi', 'India'),
    ('Todd Miranda', 'USA'),
    ('Christian Wenz', 'USA')
;

---

Books

(view table)

Id	Title
1	The Catcher in the Rye
2	Nine Stories
3	Franny and Zooey
4	The Great Gatsby
5	Tender id the Night
6	Pride and Prejudice
7	Professional ASP.NET 4.5 in C# and VB

SQL to create the table:

CREATE TABLE Books (
    Id INT NOT NULL AUTO_INCREMENT,
    Title VARCHAR(50) NOT NULL,
    PRIMARY KEY(Id)
);

INSERT INTO Books
    (Id, Title)
VALUES
    (1, 'The Catcher in the Rye'),
    (2, 'Nine Stories'),
    (3, 'Franny and Zooey'),
    (4, 'The Great Gatsby'),
    (5, 'Tender id the Night'),
    (6, 'Pride and Prejudice'),
    (7, 'Professional ASP.NET 4.5 in C# and VB')
;

---

BooksAuthors

(view table)

BookId	AuthorId
1	1
2	1
3	1
4	2
5	2
6	3
7	4
7	5
7	6
7	7
7	8

SQL to create the table:

CREATE TABLE BooksAuthors (
    AuthorId INT NOT NULL,
    BookId  INT NOT NULL,
    FOREIGN KEY (AuthorId) REFERENCES Authors(Id),
    FOREIGN KEY (BookId) REFERENCES Books(Id)
);

INSERT INTO BooksAuthors
    (BookId, AuthorId)
VALUES
    (1, 1),
    (2, 1),
    (3, 1),
    (4, 2),
    (5, 2),
    (6, 3),
    (7, 4),
    (7, 5),
    (7, 6),
    (7, 7),
    (7, 8)
;

---

Examples

View all authors (view live example):

SELECT * FROM Authors;

View all book titles (view live example):

SELECT * FROM Books;

View all books and their authors (view live example):

SELECT
  ba.AuthorId,
  a.Name AuthorName,
  ba.BookId,
  b.Title BookTitle
FROM BooksAuthors ba
  INNER JOIN Authors a ON a.id = ba.authorid
  INNER JOIN Books b ON b.id = ba.bookid
;

In this example, we have a Countries table. A table for countries has many uses, especially in Financial applications involving currencies and exchange rates.

Live example: SQL fiddle

Some Market data software applications like Bloomberg and Reuters require you to give their API either a 2 or 3 character country code along with the currency code. Hence this example table has both the 2-character ISO code column and the 3 character ISO3 code columns.

---

Countries

(view table)

Id	ISO	ISO3	ISONumeric	CountryName	Capital	ContinentCode	CurrencyCode
1	AU	AUS	36	Australia	Canberra	OC	AUD
2	DE	DEU	276	Germany	Berlin	EU	EUR
2	IN	IND	356	India	New Delhi	AS	INR
3	LA	LAO	418	Laos	Vientiane	AS	LAK
4	US	USA	840	United States	Washington	NA	USD
5	ZW	ZWE	716	Zimbabwe	Harare	AF	ZWL

SQL to create the table:

CREATE TABLE Countries (
    Id INT NOT NULL AUTO_INCREMENT,
    ISO VARCHAR(2) NOT NULL,
    ISO3 VARCHAR(3) NOT NULL,
    ISONumeric INT NOT NULL,
    CountryName VARCHAR(64) NOT NULL,
    Capital VARCHAR(64) NOT NULL,
    ContinentCode VARCHAR(2) NOT NULL,
    CurrencyCode VARCHAR(3) NOT NULL,
    PRIMARY KEY(Id)
)
;

INSERT INTO Countries
    (ISO, ISO3, ISONumeric, CountryName, Capital, ContinentCode, CurrencyCode)
VALUES
    ('AU', 'AUS', 36, 'Australia', 'Canberra', 'OC', 'AUD'),
    ('DE', 'DEU', 276, 'Germany', 'Berlin', 'EU', 'EUR'),
    ('IN', 'IND', 356, 'India', 'New Delhi', 'AS', 'INR'),
    ('LA', 'LAO', 418, 'Laos', 'Vientiane', 'AS', 'LAK'),
    ('US', 'USA', 840, 'United States', 'Washington', 'NA', 'USD'),
    ('ZW', 'ZWE', 716, 'Zimbabwe', 'Harare', 'AF', 'ZWL')
;