Abfragen
========

where exists (tupel)
where not exists (tupel)
where X < (wert)
where X < any (tupel)
where X < all (tupel)
where X in (tupel)

(case when x<1 then 'a'
      when x<2 then 'b'
      else 'c'
 end)



Integrität
==========
x integer primary key
foreign key (tuple) references table 
on delete cascade/set null, on update cascade/set null

Semester   integer check Semester between 1 and 13
Rang       char(2) check (Rang in ('C2','C3','C4'))

constraint Name
  check (exists ( select...) )


trigger -> echter code