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Writing inner joins and outer joins in SQL Part six of a six-part series of blogs
An in-depth SQL tutorial on how to create inner joins, left outer joins, right outer joins, full outer joins, cross joins and self-joins! Inner Joins, Outer Joins and Shake-it-all-about Joins Creating Joins using your Mouse - the Query Editor Writing Inner Joins in SQL Full, Left and Right Outer Joins in SQL Cross Joins using SQL Self-Joins in SQL (joining a table to itself) (this blog) This online training on joins is part of a longer tutorial on the whole of SQL. Naturally, Wise Owl also run a range of SQL training courses for individualas and (above all) for businesses.

Writing inner joins and outer joins in SQL

Part six of a six-part series of blogs

An in-depth SQL tutorial on how to create inner joins, left outer joins, right outer joins, full outer joins, cross joins and self-joins!

Inner Joins, Outer Joins and Shake-it-all-about Joins
Creating Joins using your Mouse - the Query Editor
Writing Inner Joins in SQL
Full, Left and Right Outer Joins in SQL
Cross Joins using SQL
Self-Joins in SQL (joining a table to itself) (this blog)

This online training on joins is part of a longer tutorial on the whole of SQL. Naturally, Wise Owl also run a range of SQL training courses for individualas and (above all) for businesses.

Posted by Andy Brown on 30 November 2012

Self-Joins in SQL (joining a table to itself)

A self-join is when a table contains a link to itself.

Examples of Tables which Reference Themselves

Occasionally in SQL Server you find a table which references itself. Here's an example:

In this table, each film contains a pointer to its sequel number.

Another typical example is where you have a table of employees, each of whose records contains a field pointing to that table's immediate line manager (themselves also records in the table).

Creating a Self-Join

The secret to a self-join is to create good aliases. Here's an example query for the table above showing for each book its immediate prequel and sequel:

-- show books with their prequels and sequels

SELECT

book.BookName AS Book,

prequel.BookName AS Prequel,

sequel.BookName AS sequel

FROM

tblBook AS book

LEFT OUTER JOIN tblBook AS prequel

ON prequel.SequelBookId=book.BookId

LEFT OUTER JOIN tblBook AS sequel

ON book.SequelBookId=sequel.BookId

ORDER BY

book.BookName

Here are the rows that this query would show:

The books, prequels and sequels from the above query.

Notice that by using aliases, we can effectively treat each instance of the tblBook table as if it were a different table.

Parts of this blog
Inner Joins, Outer Joins and Shake-it-all-about Joins Creating Joins using your Mouse - the Query Editor Writing Inner Joins in SQL Full, Left and Right Outer Joins in SQL Cross Joins using SQL Self-Joins in SQL (joining a table to itself) (this blog)

Some other pages relevant to the above blogs include:

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