Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE ship ( ship_id number, name text, type text, nationality text, tonnage number )
TABLE: CREATE TABLE mission ( mission_id number, ship_id number, code text, launched_year number, location text, speed_knots number, fate text )

### Question ###
Show different types of ships and the average tonnage of ships of each type.

### Accurate SQL ###

SELECT type, AVG(tonnage) FROM ship GROUP BY type