This page has 0 threads | Add post

Inner Joins, Outer Joins and Shake-it-all-about Joins

Written by Andy Brown

In this tutorial

Our Example Database
A Summary of the Joins Available
A Venn Diagram View of the Tables
Generating the Sample Database

This tutorial aims to show - in an easy-to-understand way - just about everything useful there is to know about joining tables in T-SQL (the dialect of SQL used by SQL Server).

You can represent a join between tables in a database diagram - but how can you do this in SQL?

Our Example Database

Throughout the whole of this blog, I'll use the simple database shown above. Here are the two tables in this database:


The table of 4 authors	The table of 9 books

You will notice that there are mismatches between the tables:


One author has no books	Two books have no authors

A Summary of the Joins Available

Given the example above, you can use the following types of join (explained in more detail throughout the rest of this blog):

Type of join	What it would show
Inner	The 7 books and 3 authors which match up to each other.
Left outer join	The 4 authors (together with any books they've written).
Right outer join	The 9 books (together with their authors, where found).
Cross join	The 36 possible combinations of author and book.

As we'll discover, the concepts of right and left outer join are interchangeable, but I've assumed here the tables are joined from left to right.

A Venn Diagram View of the Tables

For those who remember their Venn diagrams from schoolday maths (or math, if you're reading this in the States), here's a summary of the number of records in each table:

There are 4 authors, but one of them hasn't written any books.

Generating the Sample Database

If you want to follow through the examples in the rest of this blog, run the following script in SQL Server Management Studio to generate the Bookshop database shown above:

-- create a new database

CREATE DATABASE Bookshop

USE Bookshop

-- create table of books

CREATE TABLE [tblBook](

[BookId] [int] IDENTITY(1,1) PRIMARY KEY NOT NULL,

[BookName] [varchar](100) NOT NULL,

[AuthorId] [int] NULL,)

-- add some books into table

SET IDENTITY_INSERT [tblBook] ON

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (1, N'The Day of the Triffids', 2)

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (2, N'Girl with Dragon Tattoo', 1)

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (3, N'The Chrysalids', 2)

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (4, N'The Kraken wakes', 2)

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (5, N'The girl who Played with fire', 1)

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (6, N'Emma', 4)

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (7, N'Pride and Prejudice', 4)

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (8, N'The Hobbit', 5)

INSERT [tblBook] ([BookId], [BookName], [AuthorId]) VALUES (9, N'The Life of Pi', 6)

SET IDENTITY_INSERT [tblBook] OFF

-- create table of authors

CREATE TABLE [tblAuthor](

[AuthorId] [int] IDENTITY(1,1) PRIMARY KEY NOT NULL,

[FirstName] [varchar](50) NOT NULL,

[LastName] [varchar](50) NOT NULL

)

-- add authors

SET IDENTITY_INSERT [tblAuthor] ON

INSERT [tblAuthor] ([AuthorId], [FirstName], [LastName]) VALUES (4, N'Jane', N'Austen')

INSERT [tblAuthor] ([AuthorId], [FirstName], [LastName]) VALUES (2, N'John', N'Wyndham')

INSERT [tblAuthor] ([AuthorId], [FirstName], [LastName]) VALUES (3, N'Oliver', N'Owl')

INSERT [tblAuthor] ([AuthorId], [FirstName], [LastName]) VALUES (1, N'Stieg', N'Larsson')

SET IDENTITY_INSERT [tblAuthor] OFF

Before we start looking at the nitty-gritty of inner and outer join syntax, let's look first at a way to avoid writing any SQL altogether.

Previous
tutorial

Dates and times

Previous
part

Top of page

Next
part

Creating Joins using your Mouse - the Query Editor

Next
tutorial

Grouping data

You can learn more about this topic on the following Wise Owl courses:

This page has 0 threads

Add a new post

Online tutorials

Tutorials on SQL

Joins

Inner Joins, Outer Joins and Shake-it-all-about Joins

Our Example Database

A Summary of the Joins Available

A Venn Diagram View of the Tables

Generating the Sample Database

Head office

London

Manchester