You can call sql
on dataset:
db.select(:id).from(:some_table).where(:foo => 5).sql # => "SELECT `id` FROM `some_table` WHERE (`foo` = 5)"
For update queries you can do this:
db.from(:some_table).update_sql(:foo => 5) # => "UPDATE `some_table` SET `foo` = 5"
Some similar useful methods:
insert_sql
delete_sql
truncate_sql