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. Solution (Part 1)
CREATE VIEW Siblings AS SELECT P1.child as sib1, P2.child as sib2 FROM Par P1, Par P2 WHERE P1.parent = P2.parent and P1.child<>P2.child

4. Solution (Part 2)
CREATE VIEW DCousins AS SELECT P1.child as cous1, P2.child as cous2 FROM Par P1, Par P2, Siblings S WHERE P1.parent = sib1 and P2.parent = sib2

5. Solution (Part 3)
WITH Recursive Cousins(C1, C2) AS (SELECT * FROM DCousins) UNION (SELECT P1.child, P2.child FROM Par P1, Par P2, Cousins C WHERE P1.parent = c1 and P2.parent = c2) SELECT * FROM Cousins

6. 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
10.select * from a; select * from viewA;
What will be returned on line 6? On line 8? On line 10?

7. Solution Line 6: 13 Line 8: Line 10:
Select * from a: (1,2), (15,7), (16,19)
Select * from viewA: 15
Line 10:
Select * from a: (3,2), (15,7), (16,19)
Select * from viewA: 15, 3

8. 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;
Solution:
(2,5), (3,6), (6,3)
nums a b 1 5 2 6 3

9. 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;
Solution:
(3,5), (4,6), (6,3)
nums a b 1 5 2 6 3

10. 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;
Solution:
First:10, 21, 5, 2
Second: 27, 4
first a 10 21
second b

11. 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;
Solution:
First:10, 21
Second: 27, 4
first a 10 21
second b

12. 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;
Solution:
First:10, 21, 5, 2
Second: 27, 4
first a 10 21
second b

13. 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;
Solution:
First:10, 21
Second: 27, 4
first a 10 21
second b

14. 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;
Solution: The answer is order dependent. First will have 4,5.
We will either get:
Second with 6, 11 or
Second with 5, 12
first a 1 2
second b

15. 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;
Solution:
The first select will return 1,1,2,3,4
The second select will return 1,1,2,3,4,0
nums a 1

16. 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

17. 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);
Solution: After first insert
R: 0, S: 1, T: 1
After second insert
R: 0 , S: 0, 1, T: 1, 1 R a S a T a