/*
	App usage data are kept in the following table:
	TABLE sessions
	  id INTEGER PRIMARY KEY,
	  userId INTEGER NOT NULL,
	  duration DECIMAL NOT NULL

	  Write a query that selects userId and average session duration for each user who has more than one session.
*/

-- create a table
CREATE TABLE students (
  id INTEGER PRIMARY KEY,
  name TEXT NOT NULL,
  userId INTEGER NOT NULL,
 duration DECIMAL NOT NULL
);
-- insert some values
INSERT INTO students VALUES (1, 'Ryan', 12, 25.3);
INSERT INTO students VALUES (4, 'Ryan', 12, 25.3);
INSERT INTO students VALUES (2, 'Joanna', 15, 52.3);
INSERT INTO students VALUES (3, 'Joanna', 15, 22.3);
-- fetch some values
SELECT * FROM students ;

select name, userid, avg(duration) as avg
from students where userid in (select userid from students group by userid having count(*) > 1)
group by userid