The "standard SQL" way to do this is
Code:
CAST(my_column AS CHAR(20))
This should also be the most efficient way of doing things since CAST is not a scalar function but a built-in functionality.
Of course, most people will use a scalar function like SUBSTR, SUBSTRING or LEFT, but for one thing these names are not standardised so your SQL becomes less portable, and secondly there may be a performance hit when calling scalar functions.