why does my trigger function cause a null error? - sql

Question

PostgreSQL.
My trigger func:
CREATE OR REPLACE FUNCTION new_purchase() RETURNS TRIGGER AS $psql$
BEGIN
IF NOT EXISTS(
SELECT 1
FROM currents c
WHERE c.name = new.name
)
THEN
INSERT INTO currents (name, qty)
VALUES (new.name, new.qty);
ELSE
UPDATE currents
SET
qty = qty + new.qty
WHERE
name = new.name;
END IF;
return new;
END;
$psql$ language plpgsql;
Then my trigger:
create trigger new_purchase_trigger
after insert on purchases
for each statement
execute procedure new_purchase();
my two tables with which i am manipulating:
Purchases
CREATE TABLE purchases (
name VARCHAR,
date TIMESTAMP,
qty INTEGER,
price NUMERIC,
about VARCHAR
)
Currents
CREATE TABLE currents (
name VARCHAR,
qty INTEGER
)
My INSERT:
INSERT INTO purchases (name, date, qty, price, about)
VALUES ('cheese', '2022-07-31', 1000, 11500, 'holland');
And finally the error when i am adding data to purchases via INSERT INTO - VALUES:
ERROR: NULL value in column "name" of relation "currents" violates the NOT NULL constraint
DETAIL: Error string contains (null, null).
CONTEXT: SQL statement: "INSERT INTO currents (name, qty)
VALUES(new.name, new.qty)"
PL/pgSQL new_purchase() function, line 9, statement SQL statement
SQL state: 23502

Answer 1

You need a ROW level trigger for what you are trying to achieve. In a statement level trigger (for each statement) the record new is not populated:
create trigger new_purchase_trigger
after insert on purchases
for each row
execute procedure new_purchase();
Note that you can simplify your trigger function if you declare currents.name as the primary key:
CREATE OR REPLACE FUNCTION new_purchase()
RETURNS TRIGGER
AS $psql$
BEGIN
INSERT INTO currents (name, qty)
VALUES (new.name, new.qty)
on conflict (name) do update
set qty = currents.qty + excluded.qty;
return new;
END;
$psql$
language plpgsql;