An INNER JOIN clause allows us to join multiple tables in a single query, although it requires a specific condition in order for it to work correctly. We must ensure that the INNER JOIN statement has two tables with at least one common or overlapping field. We already know the Employee and Location tables share a common field (LocationID). The relationship is between Employee.LocationID and Location.LocationID, so we instruct SQL Server that the INNER JOIN is on this field and voila! We have combined two tables into one result set.
SQL in Sixty Seconds Video
We have attempted to explain the same subject in simple words over in following video.
Get the book for yourself and your friend. This is just a reference everyone must have it.
Read the related blog post: SQL Basics: Joining Tables – Day 3 of 10
Reference: Pinal Dave (http://blog.sqlauthority.com)