I spent a good chunk of time this morning trying to figure out how to get the views module to sort a character field with numeric data correctly. The audio module has a normalized table of meta-data meaning that there's one column for the tag name and one for the value. The value is stored as a character string which causes problem when sorting numeric data like the track numbers or years. If you've got a SELECT value FROM audio_metadata ORDER BY value that returns the range of numbers 1...13 it ends up sorted as 1,10,11,12,13,2,3...9. The trick as I discovered is to add zero to the field to coerce it to a numeric value: SELECT value + 0 AS v FROM audio_metadata ORDER BY v.

The problem then is to figure out how to get the views module to generate this bit SQL to get the sorting right. The solution I came upon is when defining the field set 'notafield' to TRUE and provide a 'query_handler' to generate the correct SQL. I've included the relevant parts of the audio module below to demonstrate how it works. You can see the complete code here.