1. Views, Triggers and Recursive Queries

2. Views and Recursive Queries
Assume we have a relation Par(child, parent).
Write a view that contains all pairs of siblings.
Write a view that contains all pairs of direct cousins.
Write a query that computes all (direct or indirect) cousins.

3. Views create table a(col1 integer, col2 integer);
create view viewA as select col1 from a where col1>col2;
insert into a values(1,2);
insert into a values(13,7);
insert into a values(16,19);
select * from viewA;
update viewA set col1 = col1 + 2 where col1<15
select * from a; select * from viewA;
update a set col1 = col1 + 2 where col1<15
What will be returned on line 6? On line 8? On line 10?

4. Triggers create or replace function mystery() returns trigger as $$
begin
new.a := (new.a + old.a)/2;
new.b := (new.b + old.b)/2;
return new;
end
$$ language plpgsql;
create trigger mysterytrig
before update on nums
for each row
execute procedure mystery();
update nums set a = a+2 where b>4;
select * from nums;
nums a b 1 5 2 6 3

5. Triggers create or replace function mystery() returns trigger as $$
begin
new.a := (new.a + old.a)/2;
new.b := (new.b + old.b)/2;
return new;
end
$$ language plpgsql;
create trigger mysterytrig
after update on nums
for each row
execute procedure mystery();
update nums set a = a+2 where b>4;
select * from nums;
nums a b 1 5 2 6 3

6. Triggers create or replace function enigma() returns trigger as $$
begin
insert into second values(new.A * new.A);
update second set b=b+1 where b>12;
return null;
end
$$ language plpgsql;
create trigger enigmatrig
after insert on first
for each row
execute procedure enigma();
insert into first values(5); insert into first values(2);
select * from first; select * from second;
first a 10 21
second b

7. Triggers create or replace function enigma() returns trigger as $$
begin
insert into second values(new.A * new.A);
update second set b=b+1 where b>12;
return null;
end
$$ language plpgsql;
create trigger enigmatrig
before insert on first
for each row
execute procedure enigma();
insert into first values(5); insert into first values(2);
select * from first; select * from second;
first a 10 21
second b

8. Triggers create or replace function enigma() returns trigger as $$
begin
insert into second values(new.A * new.A);
update second set b=b+1 where b>12;
return new;
end
$$ language plpgsql;
create trigger enigmatrig
before insert on first
for each row
execute procedure enigma();
insert into first values(5); insert into first values(2);
select * from first; select * from second;
first a 10 21
second b

9. Triggers create or replace function enigma() returns trigger as $$
begin
insert into second values(new.A * new.A);
update second set b=b+1 where b>12;
return old;
end
$$ language plpgsql;
create trigger enigmatrig
before insert on first
for each row
execute procedure enigma();
insert into first values(5); insert into first values(2);
select * from first; select * from second;
first a 10 21
second b

10. Triggers create or replace function enigma() returns trigger as $$
begin
insert into second values(old.A * new.A);
update second set b=b+1;
return old;
end
$$ language plpgsql;
create trigger enigmatrig
after update on first
for each row
execute procedure enigma();
update first set a = a+3;
select * from first; select * from second;
first a 1 2
second b

11. Triggers create or replace function what() returns trigger as $$
declare
r nums%ROWTYPE;
begin
select max(a) into r from nums;
IF r.a<new.a
THEN insert into nums values(new.a-1);
END IF;
RETURN new;
end
$$ language plpgsql;
create trigger whattrig
before insert on nums
for each row
execute procedure what();
insert into nums values(4); select * from nums;
insert into nums values(0); select * from nums;
nums a 1

12. Triggers create or replace function whatr() returns trigger as $$
begin
insert into S values(1);
RETURN null;
end
$$ language plpgsql;
create or replace function whats() returns trigger as $$
insert into T values(1); R a S a T a

13. Triggers (cont) create trigger whatstrig before insert on S
for each statement
execute procedure whats();
create trigger whatrtrig
before insert on R
execute procedure whatr();
insert into R values(0);
select * from R; select * from S; select * from T;
insert into S values(0); R a S a T a