While it doesn't happen that often, occasionally I'll need to export a certain subset of data from a MySQL table instead of the entire dataset. You can do this with mysqldump using the --where option which allows you to apply a WHERE clause that filters the data in your tables based on a certain condition.
Let's say in Drupal, you'd like to copy a single comment from one table into another. Here's how you'd get an insert statement for comment ID 55:
mysqldump -u user -p --where="cid=55" dbname table_name LOCK TABLES `comments` WRITE; INSERT INTO `comments` VALUES (4,0,2748,5,'this is a comment dude.','This is a comment.','216.555.252.21',1271092388,1,1,'01/','david smith','',''); UNLOCK TABLES;
Now you can run this INSERT statement on the database you'd like to copy this record to, and you're done.