Trigger function updates all rows instead of one (PostgreSQL) - sql

Question

Using PostgreSQL.
What I have is:
3 tables
1.Customer2c with columns: CustomerID,PersonID,Number_Of_Items`.
2.SalesOrderHeader2c with columns: SalesOrderID,CustomerID.
SalesOrderDetail2c with columns: SalesOrderDetailID,SalesOrderID,OrderQty
I want to create a trigger function, that will trigger whenever someone uses
INSERT INTO 'SalesOrderDetail2c' table
and that is going to get the OrderQty that was inserted and update the correspondent Number_Of_Items field with it.
My trigger is working, but the problem is that whenever I insert a new value to the SalesOrderDetail2c, the function gets the OrderQty value and updates all the rows of Number_Of_Items with it, instead of updating just the correspondent one.
Any help appreciated. What I have so far is this (It may be copletely wrong, dont judge please!):
CREATE OR REPLACE FUNCTION FunctionTrigger2c() RETURNS TRIGGER AS
$BODY$
BEGIN
UPDATE Customer2c
SET Number_Of_Items =
(SELECT OrderQty
FROM SalesOrderDetail2c
INNER JOIN SalesOrderHeader2c ON (SalesOrderDetail2c.SalesOrderID = SalesOrderHeader2c.SalesOrderID)
INNER JOIN Customer2c ON (SalesOrderHeader2c.CustomerID = Customer2c.CustomerID)
ORDER BY SalesOrderDetailID DESC LIMIT 1
)
FROM SalesOrderHeader2c
WHERE SalesOrderHeader2c.CustomerID = Customer2c.CustomerID
;
RETURN NEW;
END;
$BODY$
language plpgsql;
CREATE TRIGGER Trigger2c
AFTER INSERT ON SalesOrderDetail2c
FOR EACH ROW
EXECUTE PROCEDURE FunctionTrigger2c();

Answer 1

I had to use .new as #Nicarus mentioned above! Thanks again by the way.
This is the new code and now it changes only the correspondent value.
CREATE OR REPLACE FUNCTION FunctionTrigger2c() RETURNS TRIGGER AS
$BODY$
BEGIN
UPDATE Customer2c
SET Number_Of_Items =
(SELECT new.OrderQty
FROM SalesOrderDetail2c
order by salesorderdetailid desc limit 1
)
FROM SalesOrderheader2c
WHERE (SalesOrderheader2c.salesorderID = new.salesorderID) and (salesorderheader2c.customerid = customer2c.customerid)
;
RETURN NEW;
END;
$BODY$
language plpgsql;
CREATE TRIGGER Trigger2c
AFTER INSERT ON SalesOrderDetail2c
FOR EACH ROW
EXECUTE PROCEDURE FunctionTrigger2c();

Answer 2

I am pointing out why your original trigger did not work. The reason it updated every row is because of your UPDATE statement. After you inserted something, it triggered the update.
When the update ran it got these things:
What table I am gonna update
What value am I gonna set on what column of the given table
What rows I am gonna implement this update on (FROM + WHERE)
The problem lies in the last part, basically calling FROM in an UPDATE statement is like calling a SELECT statement, it tried to process every row from the FROM clause tables with the given WHERE value.
PostgreSQL documentation about UPDATE statement from_list parameter states that "A
list of table expressions, allowing columns from other tables to
appear in the WHERE condition and the update expressions. This is
similar to the list of tables that can be specified in the FROM Clause
of a SELECT statement."
(https://www.postgresql.org/docs/9.1/sql-update.html)
Basically, your update statement went through every row in SalesOrderHeader2c which conveniently matched with every row of Customer2c (with SalesOrderHeader2c.CustomerID = Customer2c.CustomerID), updating the value of every row in Customer2c with the firstly found Number_Of_Items value from the UPDATE SET statement. This is why you got the same value for every row in Customer2c.
The reason the new trigger works is because the update statement is done with the correctly selected rows.

Answer 3

Here is my, quite general trigger that is used do count/sum values in dependent tables. You should take care of all order changes, ie not only INSERT but also DELETE and UPDATE. You also should read #Nicarus comment about using NEW (and OLD) in triggers.
I have altered it to match your schema, but didn't test it...
CREATE OR REPLACE FUNCTION FunctionTrigger2c()
RETURNS trigger AS
$BODY$
DECLARE
BEGIN
CASE TG_OP
WHEN 'INSERT' THEN
UPDATE
Customer2c
SET
Number_Of_Items = Number_Of_Items + NEW.OrderQty
FROM
SalesOrderHeader2c
WHERE
Customer2c.CustomerID = SalesOrderHeader2c.CustomerID AND
SalesOrderHeader2c.SalesOrderID = NEW.SalesOrderID AND
SalesOrderDetailID = NEW.SalesOrderDetailID;
WHEN 'UPDATE' THEN
UPDATE
Customer2c
SET
Number_Of_Items = Number_Of_Items - OLD.OrderQty
FROM
SalesOrderHeader2c
WHERE
Customer2c.CustomerID = SalesOrderHeader2c.CustomerID AND
SalesOrderHeader2c.SalesOrderID = OLD.SalesOrderID AND
SalesOrderDetailID = OLD.SalesOrderDetailID;
UPDATE
Customer2c
SET
Number_Of_Items = Number_Of_Items + NEW.OrderQty
FROM
SalesOrderHeader2c
WHERE
Customer2c.CustomerID = SalesOrderHeader2c.CustomerID AND
SalesOrderHeader2c.SalesOrderID = NEW.SalesOrderID AND
SalesOrderDetailID = NEW.SalesOrderDetailID;
WHEN 'DELETE' THEN
UPDATE
Customer2c
SET
Number_Of_Items = Number_Of_Items - OLD.OrderQty
FROM
SalesOrderHeader2c
WHERE
Customer2c.CustomerID = SalesOrderHeader2c.CustomerID AND
SalesOrderHeader2c.SalesOrderID = OLD.SalesOrderID AND
SalesOrderDetailID = OLD.SalesOrderDetailID;
END CASE;
RETURN NULL;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
UPDATE also work well if you change the SalesOrderID :)
Notice, that this trigger returns NULL and it should be set as AFTER INSERT OR UPDATE OR DELETE:
CREATE TRIGGER Trigger2c
AFTER INSERT OR UPDATE OR DELETE
ON SalesOrderDetail2c
FOR EACH ROW
EXECUTE PROCEDURE FunctionTrigger2c();