INNER JOIN: Returns all rows when there is at least one match in BOTH tables
LEFT JOIN: Return all rows from the left table, and the matched rows from the right table
RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table
FULL JOIN: Return all rows when there is a match in ONE of the tables
SELECT class.st_person_id,class.name, householder.address
FROM class
LEFT JOIN householder
ON class.st_person_id=householder.st_person_id
where class.now_class='201';