Hello, and welcome to types of relationships.

In this video, we will provide an overview of

the relationships that exist between entities.

At the end of this video,

you will be able to describe the building blocks of a relationship,

explain the symbols used in a relationship set,

and describe the difference between the one-to-one and many-to-many relationship.

The building blocks of a relationship are: entities,

relationship sets, and crows foot notations.

Entity sets are represented by a rectangle.

Relationship sets are represented by a diamond,

with lines connecting associated entities.

Different techniques are used in representing relationships.

For ease of understanding,

this lesson uses the crows foot notations.

Some of these are the greater than symbol,

the less than symbol and a vertical line.

An entity relationship diagram or ER diagram for the entity book would look like this,

the entity book is drawn as a rectangle and the attributes are drawn as ovals.

Attributes are certain properties of that entity,

for example title, edition, year, price, etcetera.

Attributes are connected to exactly one entity.

For the entity author,

the ER diagram would look like this,

the entity author has attributes: last name,

first name, email, city,

country, and author ID.

Let's see how the entitys' book and author relate to each other.

A book must be written by at least one author.

However, a book can be written by two authors,

and a book can be written by many authors.

As another example one author can write just one book,

or two books, or multiple books.

In both cases there is a relationship between the book and the author.

In our examples we see that the book needs at least an author.

The relationship set that comes here is authored by,

one book must be written by one author.

The thick lines indicate each entity in the entity set

is involved in at least one and exactly one relationship.

This is called a one-to-one relationship.

Only entities are used in the relationship diagrams.

Attributes are omitted because they can clutter the diagrams.

Earlier we saw that more than one author can write a book.

This can be represented with the different notation called crow's foot notation.

In this case a less than symbol.

This indicates that one book entity is

participating in more than one relationship in the relationship set.

This is called a one-to-many relationship.

This could also be called a many-to-one relationship,

in that many authors write a single book.

To represent many authors writing many books use the

greater than and less than symbols on either side of the relationship set.

This is called a many-to-many relationship.

Each entity in the entity set is participating in more than one relationship.

Many books being written by many authors,

or many authors writing many books.

You can now describe the building blocks of a relationship,

explain the symbols used in a relationship set,

and describe the difference between the one-to-one and many-to-many relationship.

Thanks for watching this video.