Table: ship_flotilla
Columns: vessel
origin
type
in_service
unit

Question: What type of ship is a Kobben class vessel that has been in service longer than 2?
SQL: SELECT "type" FROM ship_flotilla WHERE "in_service" > 2 AND "vessel" = 'kobben class'

Question: What is the unit for a submarine type ship from Norway?
SQL: SELECT "unit" FROM ship_flotilla WHERE "type" = 'submarine' AND "origin" = 'norway'