Joins start off as a cross-product, and predicates ("ON") filter the rows down.

• LEFT OUTER JOIN - always return all rows for A, with anything that happens to match in B (ie. B could be null).
• RIGHT OUTER JOINs are fairly useless - just do a left join for intuitiveness (possible exception for 3+ tables?).
• FULL OUTER JOIN - either side could be null.
• INNER JOIN - return only rows with matching records on both sides.