One big query vs. lots of small queries

Here's a question of efficiency. I have an application in PHP using MySQL. Users select items on a form and each selected item represents a change in a table row. It is possible that 100s of changes might be chosen from a single form. Which is likely to be faster/more efficient? multiple queries like... $query="UPDATE some_table SET x='$y' WHERE id='$z[$key]'"; $result=mysql_query($query); or one huge query like... $query="UPDATE some_table SET x='$y' WHERE id='$z[$key]' OR id='$z[$another_key]' OR id='$z[$another_key] OR id='$z[$another_key]"; etc? $result=mysql_query($query); In the latter, PHP would have to construct the WHERE string, which would require some processing power as well. Is there a limit to how many OR's can be added to a mysql query? No - don't answer that, I'll look in the manual. Many thanks, Jeff