What are the differences between LEFT, RIGHT, INNER and OUTER JOINS?
An INNER JOIN is the most common type: It combines all the data from the tables specified and eliminates the rows that do not meet the criteria specified in the WHERE clause.
LEFT and RIGHT JOINs are types of OUTER JOINs. In general, an OUTER JOIN is one that preserves all the rows in one of the tables specified in the FROM clause, even if there are no matching rows with the other table. A LEFT OUTER JOIN means that the preserved table is the one specified on the left side of the equation; a RIGHT OUTER JOIN means that the preserved table is the one specified on the right side of the equation. A FULL OUTER JOIN preserves the tables on both sides of the equation.