I literally always have to look up the meaning of :limit in migrations when it comes to integer values. Here’s an overview. Now let’s memorise it (oh, this works for MySQL, other databases may work differently):
First seen at Kabisa Blog: SQL: Ordering with NULL values
This post tells you how to sort NULL values in a column to the bottom and sort the remaining non-NULL values.
This is really great in combination with LEFT JOIN queries that may yield NULL values.