Understanding Database Joins

Craig Courtney writes a brief summary of the differences between INNER, LEFT and RIGHT joins in SQL.

An INNER JOIN returns only those rows from left table where they have a matching row in right table based on the join criteria.

A LEFT JOIN returns ALL rows from the left table even if no matching rows where found in the right table. Any values selected out of the right table will be null for those rows where no matching row is found in the right table.

A RIGHT JOIN works exactly the same as a left join but reversing the direction. So it would return all rows in the right table regardless of matching rows in the left table. It is recommended that you no use right joins as your query can always be rewritten to use left joins which tend to be more portable and easier to read.

Note: INNER joins are faster than LEFT joins at least according to the tests i've read about.