How do I update a summary table with a trigger? - postgresql

Question

I am working with the sample DVD_Rental database. I have to create a trigger on my category_performance_details table that will create a summary table every time data is added to the detail table.
These are the queries I am using to setup my tables:
DROP TABLE IF EXISTS category_performance_summary;
CREATE TABLE category_performance_summary (
genre VARCHAR(25),
total_sales numeric
);
DROP TABLE IF EXISTS category_performance_details;
CREATE TABLE if not exists category_performance_details (
category_id Int,
category_name VARCHAR(25),
film_id Int,
film_title VARCHAR(255),
film_rental_rate numeric(4,2),
inventory_id Int,
payment_id Int,
payment_amount numeric(4,2),
rental_id Int,
rental_date Timestamp
);
I want to update the summary table after every insert statement with a trigger so that it provides a summary of the total sales per category. Basically, the summary table should be the same result as:
SELECT category_name, SUM(payment_amount) FROM category_performance_details
GROUP BY category_name;
This is my procedure and trigger. For some reason, I am getting a syntax error at or near "CREATE TRIGGER". Am I approaching this trigger correctly? What is wrong with my syntax?
CREATE OR REPLACE FUNCTION update_summary_table()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS
$$
BEGIN
TRUNCATE TABLE category_performance_summary;
INSERT INTO category_performance_summary (genre, total_sales)
SELECT category_name, SUM(payment_amount)
FROM category_performance_details
GROUP BY category_name;
RETURN NEW;
END;
$$
CREATE TRIGGER update_summary
AFTER INSERT
ON category_performance_summary
FOR EACH STATEMENT
EXECUTE PROCEDURE update_summary_table();

Answer 1

You can also simply increment or decrement the total_sales in the trigger function.
In any case you have to use the NEW (resp. OLD) key word in order to refer to the values that are inserted/updated (resp. deleted) in table category_performance_summary :
CREATE OR REPLACE FUNCTION increment_summary_table()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS
$$
BEGIN
INSERT INTO category_performance_summary (genre, total_sales)
VALUES (NEW.category_name, 1)
ON CONFLICT (genre) DO UPDATE
SET total_sales= total_sales + 1
WHERE genre= NEW.category_name ;
RETURN NEW;
END;
$$ ;
CREATE TRIGGER increment_summary
AFTER INSERT
ON category_performance_summary
FOR EACH STATEMENT
EXECUTE PROCEDURE increment_summary_table();
CREATE OR REPLACE FUNCTION decrement_summary_table()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS
$$
BEGIN
UPDATE category_performance_summary
SET total_sales= total_sales - 1
WHERE genre= OLD.category_name ;
RETURN OLD;
END;
$$ ;
CREATE TRIGGER decrement_summary
AFTER DELETE
ON category_performance_summary
FOR EACH STATEMENT
EXECUTE PROCEDURE decrement_summary_table();