You use LEFT JOIN when you absolutely must have all the columns from the left hand table that match the criterial in your WHERE, but might not exist in the right hand table. E.g you have a DVD renting shop, want a list of all users that a doctors and what, if any, DVDs they have out.
You'd use
SELECT users.name, dvds.title FROM users LEFT JOIN dvds ON users.id = dvds.hiredby WHERE users.title = 'Dr';
Some of the rows returned will have no DVD titles, but you'll still know if Dr. Shipman hasn't returned that copy of 'Cocoon' or not.
If you just used the standard 'comma' join you'd only get users that have DVDs out.
And if you wanted to find doctors that have no DVDs out you can add AND dvds.title IS NULL on the end of the above query, as NULL is what the empty fields will be when there is no match on the right.
