CODEWITHSUNDEEP
postgresqltriggersplpgsql
knight
knight

Reputation: 65

Postgres Trigger - Working with 2 different tables

I have 2 tables:

Books - isbn (PK), title, qty-in-stock
Orderlist - ordernum (PK), isbn, quantity

My goal is to insert a record into orderlist and then, if matching isbns, add the quantity to qty-in-stock

My function and trigger is not correct -

CREATE OR REPLACE FUNCTION books_upd() RETURNS trigger as $bookupd$
BEGIN 
--IF   THE ISBN MATCHES BETWEEN BOOKS AND ORDERLIST
-- ADD THE EXISTING QTY_IN_STOCK (BOOKS)  TO QUANTITY (ORDERLIST)
QTY_IN_STOCK:=QTY_IN_STOCK+QUANTITY;
--END IF
RETURN NEW;
$bookupd$ LANGUAGE plpgsql;

CREATE TRIGGER books_upd
BEFORE INSERT OR UPDATE on orderlist
FOR EACH ROW
EXECUTE PROCEDURE books_upd();

Can anyone help?

Upvotes: 4

Views: 183

Answers (1)

klin
klin

Reputation: 121574

The trigger function is quite complicated because of different cases of insert and update.

When a user updates the order, then the function should:

  • add to qty_in_stock the difference between new and old quantities if isbns are the same or
  • subtract old quantity from qty_in_stock of one book and add new quantity to qty_in_stock of another book when isbns are different.

Additionally, the function should reject changes if the given isbn does not exist in books.

CREATE OR REPLACE FUNCTION books_upd() 
RETURNS trigger as $bookupd$
DECLARE
    v_quantity int;
BEGIN
    v_quantity = NEW.quantity;
    IF TG_OP = 'UPDATE' THEN
        IF OLD.isbn != NEW.isbn THEN
            UPDATE books
            SET qty_in_stock = qty_in_stock- OLD.quantity
            WHERE isbn = OLD.isbn;
        ELSE
            v_quantity = NEW.quantity- OLD.quantity;
        END IF;
    END IF;

    UPDATE books
    SET qty_in_stock = qty_in_stock+ v_quantity
    WHERE isbn = NEW.isbn;

    IF NOT FOUND THEN
        RAISE EXCEPTION 'Uknown isbn';
    END IF;
    RETURN NEW;
END;
$bookupd$ LANGUAGE plpgsql;

Read more about NEW, OLD and TG_OP.

The trigger function only for insert is really simple:

CREATE OR REPLACE FUNCTION books_upd() 
RETURNS trigger as $bookupd$
BEGIN
    UPDATE books
    SET qty_in_stock = qty_in_stock+ NEW.quantity
    WHERE isbn = NEW.isbn;

    RETURN NEW;
END;
$bookupd$ LANGUAGE plpgsql;

Upvotes: 1

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