Postgres - Trigger with matched key - postgresql

Question

I have several tables. A table, cexp, is a table that has attributes cid and total. Cid is grouped and total is the sum of quantity * price for that cid (matched on cid)
The cexp table was populated with the results of the following code:
SELECT c.cid, sum(ol.quantity*b.price) as total
FROM customers c join orders o on c.cid=o.cid
join orderlist ol on o.ordernum=ol.ordernum
join books b on b.isbn=ol.isbn
GROUP BY C.CID
My task is to create a trigger that, when inserting rows for order and orderderlist, finds the matching name, in cexp and increments the existing total by the product of new quantity (from orderlist) and the price (from books). If no match, insert a row in cexp.
Tables are as follows:
Customers-cid,name pk-cid
Books - isbn,title,price pk-isbn
Orders - ordernum,cid pk-ordernum
Orderlist - ordernum,isbn, quantity - pk-(ordernum,isbn)
cexp - cid,total - pk-cid
I am getting syntax errors. Can anyone correct this code?
CREATE OR REPLACE FUNCTION cexpupd()
RETURNS trigger as
$cexpupd$
BEGIN
UPDATE cexp
SET new.total=total+Select (b.price*new.quantity) FROM customers c
join orders o on c.cid=o.cid
join orderlist ol on o.ordernum=ol.ordernum
join books b on b.isbn=ol.isbn
where b.isbn=new.isbn;
--INSERT CODE WHEN ABOVE LINE DOES NOT OCCUR -INSERTS NEW ROW INTO CEXP
END;
$cexpupd$
LANGUAGE plpgsql

Answer 1

I would say that your UPDATE statement is an unsalvageable birds-nest. Fortunately, there's an easier way to achieve the same result.
Keep in mind that a trigger function is procedural code, so there's no need to concurrently load the gun, pull the trigger, scream, and shoot yourself in the foot in a single statement. You have access to all the procedural goodies like local variables and flow control statements.
The following should give you a good headstart on what you want:
CREATE OR REPLACE FUNCTION cexpupd()
RETURNS trigger as
$cexpupd$
DECLARE
bookprice books.price%TYPE;
ext_total cexp.total%TYPE;
custid orders.cid%TYPE;
BEGIN
SELECT cid
INTO custid
FROM orders
WHERE orders.ordernum = NEW.ordernum;
SELECT price
INTO bookprice
FROM books
WHERE books.isbn = NEW.isbn;
ext_total = bookprice * NEW.quantity;
UPDATE cexp
SET total = ext_total
WHERE cid = custid;
IF NOT FOUND THEN
--INSERT new CID record here
INSERT INTO cexp
(cid, total)
VALUES
(custid, ext_total);
END IF;
RETURN NEW;
END;
$cexpupd$
LANGUAGE plpgsql;
Note the statement near the end, RETURN NEW; This line is crucial, as it is how a PostgreSQL trigger function tells the database to go ahead and finish executing the statement that fired the trigger.
If you need any clarification, please don't hesitate to ask. Please note I have not tried executing this function, but I did create the necessary tables to compile it successfully.