How to retrieve number of results of MySQL result in PHP?

$sql = 开发者_如何学Go'select ...';

$result = mysql_query($sql,$con);

Is there a function for this?

mysql_num_rows()

Retrieves the number of rows from a result set. This command is only valid for statements like SELECT or SHOW that return an actual result set. To retrieve the number of rows affected by a INSERT, UPDATE, REPLACE or DELETE query, use mysql_affected_rows().