SELECT E Career.name, m1.name AS Subject, Plan.credits, Plan.h_teoric, Plan.h_pract, m2.name AS Link

FROM Career, Plan, Subject m1, Subject m2

WHERE Plan.id_career = Career.id_career
AND Plan.id_subject = m1.id_subject AND
Plan.link = m2.id_subject

IN THE TABLE Plan, link field CAN BE NULL, I NEED TO SHOW THOSE FIELDS TOO IN THIS QUERY