Is it possible to use MySQL's SUBSTRING() function in a WHERE clause?

To my surprise my queries return no results when I try to do this, and I can't find any information on whether or not SUBSTRING() can be used in the WHERE clause. I simply want to return results only where the first letter of the value of a column matches a certain letter. Maybe there's a more proper way to do it?

 

Try something like this:

SELECT * FROM table WHERE field LIKE 'aa%';