Hvordan implementerer man én-til-en, én-til-mange og mange-til-mange relationer, mens man designer tabeller?

En-til-en: Brug en fremmednøgle til den refererede tabel:

student: student_id, first_name, last_name, address_id
address: address_id, address, city, zipcode, student_id # you can have a
                                                        # "link back" if you need

Du skal også sætte en unik begrænsning på kolonnen med fremmednøgle (addess.student_id ) for at forhindre flere rækker i den underordnede tabel (address ) fra at relatere til den samme række i den refererede tabel (student ).

En-til-mange :Brug en fremmednøgle på den mange side af forholdet, der linker tilbage til "en"-siden:

teachers: teacher_id, first_name, last_name # the "one" side
classes:  class_id, class_name, teacher_id  # the "many" side

Mange-til-mange :Brug en krydstabel (eksempel):

student: student_id, first_name, last_name
classes: class_id, name, teacher_id
student_classes: class_id, student_id     # the junction table

Eksempelforespørgsler:

 -- Getting all students for a class:

    SELECT s.student_id, last_name
      FROM student_classes sc 
INNER JOIN students s ON s.student_id = sc.student_id
     WHERE sc.class_id = X

 -- Getting all classes for a student: 

    SELECT c.class_id, name
      FROM student_classes sc 
INNER JOIN classes c ON c.class_id = sc.class_id
     WHERE sc.student_id = Y