trigger insert only if tupel satisfies condition - sql

Question

I want to create a trigger which checks before insert if the tupel which is supposed to be inserted holds a specific condition (which also depends on another table).
For example:
create trigger or replace check_tupel
before insert on A
for each row
execute
if exists (select x,y from B where B.x = A.x and B.y = A.y)
Oh I am using postgreSQL 13.
EDIT: Yes I know that I can do this without a trigger, but I am asking for a solution with a trigger for a reason.
I hope there is a way to do this... My other idea was to create a UDF which gets called before insert but I do not know how to check the condition in this UDF and only insert if the function returns true.

Answer 1

If you simply wanna automatically validate a record before inserting it on table A based on table B using a User Defined Function, you do not need a trigger at all. Consider adding a simple CHECK CONSTRAINT:
CREATE TABLE a (
x int,
y int,
CONSTRAINT exists_in_b CHECK (NOT myfunc(x,y))
);
Demo: db<>fiddle
CREATE TABLE b (x int,
y int);
INSERT INTO b VALUES (42,42);
CREATE OR REPLACE FUNCTION myfunc(x int, y int)
RETURNS BOOLEAN AS $BODY$
SELECT EXISTS (SELECT 1 FROM b WHERE b.y =$1 AND b.x=$2 )
$BODY$
LANGUAGE sql;
CREATE TABLE a (
x int,
y int,
CONSTRAINT exists_in_b CHECK (NOT myfunc(x,y)) -- here the magic happens
);
Now, if we try to insert a value that our function does not validate, it raises an exception:
INSERT INTO a VALUES (42,42);
ERROR: new row for relation "a" violates check constraint "exists_in_b"
DETAIL: Failing row contains (42, 42).
SQL state: 23514
EDIT (See comments): Solution using a trigger
CREATE OR REPLACE FUNCTION myfunc()
RETURNS TRIGGER AS $BODY$
BEGIN
IF EXISTS (SELECT 1 FROM b WHERE b.y =NEW.y AND b.x=NEW.x ) THEN
RAISE EXCEPTION 'tuple already exists in "b": % %', NEW.x,NEW.y;
END IF;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql;
CREATE TRIGGER check_tupel
BEFORE INSERT OR UPDATE ON a
FOR EACH ROW EXECUTE PROCEDURE myfunc();
Demo: db<>fiddle

Answer 2

So you need a trigger solution, sounds like a homework problem. The question then becomes do you want to:
abort the entire operation
slightly squash the row but continue the remainder of the operation.
The following does the second: (See demo)
create or replace
function check_b_has_a()
returns trigger
language plpgsql
as $$
begin
if exists
( select null
from b
where (b.x,b.y) =
(new.x, new.y)
)
then
return null;
else
return new;
end if;
end;
$$;
create trigger a_bir
before insert
on a
for each row
execute function check_b_has_a();