Each row in the left hand table (LHT) can be associated with 0 or more rows in the right hand table (RHT).
This query returns the max value from RHT or NULL.
LHT:
account
|------------|
|id |  name  |
|------------|
|1  |  Chris |
|2  |  Mark  |
|3  |  Robert|
|------------|
RHT:
payment
|------------------------------|
|id |  accountid | paymentdate |
|------------------------------|
|1  |  1         | 2012-01-01  |
|2  |  1         | 2012-02-01  |
|3  |  2         | 2012-01-01  |
|------------------------------|
The query:
SELECT a.name,MAX(paymentdate) FROM account a LEFT JOIN payment p ON a.id=p.accountid GROUP BY a.id;
Result:
|--------------------------|
|name   | MAX(paymentdate) |
|--------------------------|
|Chris  | 2012-02-01       |
|Mark   | 2012-01-01       |
|Robert | NULL             |
|--------------------------|
Thanks to Mark Simmons for this one!