Depends on what you want to use the result for, just a check to see if the row was inserted for example bool is fine. However if you needed to check how many rows were updated, then use mysql_affected_rows.

For example, if you want to update all rows with id's between 8 and 9, but the table only holds 4 records, (thus only has id's 1-4) mysql_query will return true (i.e. mysql didnt explode), but mysql_num_rows will return 0.

In short, it depends on what you want to check for.

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)