-- create a table
CREATE TABLE students (
  id INTEGER PRIMARY KEY,
  name TEXT NOT NULL,
  gender TEXT NOT NULL,
  age INTEGER(11)
);
-- insert some values
INSERT INTO students VALUES (1, 'Ryan', 'M',10);
INSERT INTO students VALUES (2, 'Joanna', 'F',10);
-- fetch some values
(SELECT id FROM students WHERE age > 20
    ORDER BY id desc limit 1)
    UNION ALL
SELECT count(id) FROM students WHERE age < 20 AND NOT EXISTS (SELECT * FROM students WHERE age > 20  ORDER BY id desc limit 1)