PostgreSQL select sum inside procedure - sql

Question

I want a procedure that checks, which buyer spent more than 1600$ and than I want to print out their info, so I could send them a gift card.
I get error: SQL Error [42601]: ERROR: query has no destination for result data
when I call this procedure:
create or replace procedure bookstore.procedure1 ()
language plpgsql
as $$
declare
i integer := 0;
temp_price numeric;
temp_sum numeric;
temp_foreign_key integer;
number_of_buyers integer := (select count(*) from bookstore.buyer);
begin
while i < number_of_buyers loop
select sum (price) as temp_sum from bookstore.receipt where id_buyer = i;
if temp_sum > 1600 then
select id_buyer, name, surname, adress from bookstore.buyer where id_buyer = i;
end if;
i := i+1;
end loop;
end
$$;
The problem is in select sum (price) as temp_sum... row. I have the similar procedure in MySQL and it works. I tried running that row with id_buyer = 20 for example and it worked. How should I change it?
That's my first question on stackoverflow, I hope it's understandable.

Answer 1

Let SQL do all the work of serving the data. This can be done in a single statement.
select buy.buy_id
, buy.name
, buy.address
, buy.city
, buy.postal_code
, pur.purchases
from buyers buy
join ( select inv.buy_id
, sum(inv.amount) purchases
from invoices inv
group by buy_id
having sum(inv.amount) >= 1600.00
) pur
on (pur.buy_id = buy.buy_id) ;
If you must have a stored program, then wrap the above query in a SQL function that returns a table. then Select from the function.
create or replace
function preferred_buyers()
returns table ( buy_id integer
, name text
, address text
, city text
, postal_code text
, purchases numeric(7,2)
)
language sql
as $$
select buy.buy_id
, buy.name
, buy.address
, buy.city
, buy.postal_code
, pur.purchases
from buyers buy
join ( select inv.buy_id
, sum(inv.amount) purchases
from invoices inv
group by buy_id
having sum(inv.amount) >= 1600.00
) pur
on (pur.buy_id = buy.buy_id) ;
$$;
select * from preferred_buyers();
Neither of the above handles printing task. This is not something SQL very good at. Actually it cannot do so. Printing requires a programming language extension; for Postgres it is plpgsql. Also realize any printing done is not available at least in a production environment; it would be on the DB server. Handle your printing in the presentation manager (app).

Answer 2

You probably need to add the return statement
In the Function creation definition SQL, there must be a row written as RETURNS integer/text
Try-
language plpgsql
as $$
declare
i integer := 0;
temp_price numeric;
temp_sum numeric;
temp_foreign_key integer;
number_of_buyers integer := (select count(*) from bookstore.buyer);
begin
while i < number_of_buyers loop
select sum (price) as temp_sum from bookstore.receipt where id_buyer = i;
if temp_sum > 1600 then
select id_buyer, name, surname, adress from bookstore.buyer where id_buyer = i;
end if;
i := i+1;
end loop;
return 1;
end
$$;
Or else you can remove the initial statement of RETURNS text/integer by using CREATE OR REPLACE FUNCTION your_function ...

Answer 3

In plpgsql SELECT statement must have a receiver.
You must redirect the SELECT output with INTO or use direct affectation
--Fist version
SELECT SUM(price) FROM bookstore.receipt WHERE id_buyer = i INTO temp_sum;
--Second version, keep surrounding parenthesis
temp_sum = (SELECT SUM(price) FROM bookstore.receipt WHERE id_buyer = i);
Regarding your loop, you should use something more efficient and safe
DECLARE
temp_sum NUMERIC;
v_buyer bookstore.BUYER;
BEGIN
FOR v_buyer IN SELECT * FROM bookstore.buyer LOOP
--Use coalesce to manage empty result
temp_sum = (SELECT coalesce(SUM(price), 0) FROM bookstore.receipt WHERE id_buyer = v_buyer.id);
IF temp_sum > 1600 THEN
-- Your business logic
END IF;
END LOOP;
END
If your dataset is large it's better to use a cursor
DECLARE
temp_sum NUMERIC;
v_buyer_cursor CURSOR FOR SELECT * FROM bookstore.buyer;
v_buyer bookstore.BUYER;
BEGIN
FOR v_buyer IN v_buyer_cursor LOOP
temp_sum = (SELECT COALESCE(SUM(price), 0) FROM bookstore.receipt WHERE id_buyer = v_buyer.id);
IF temp_sum > 1600 THEN
-- Your business logic
END IF;
END LOOP;
END

Answer 4

None of the answers helped. I had to replace this row:
SELECT id_buyer, name, surname, adress FROM bookstore.buyer WHERE id_buyer = i;
with this row:
INSERT INTO bookstore.temp SELECT id_buyer, name, surname, adress FROM bookstore.buyer WHERE id_buyer = i;
Now it works.