How to debug PostgreSQL trigger error "query has no destination for result data"? - postgresql

Question

I have three table, like this:
data_buku table
+----+----------+----------------+
| kode_buku | * | * | stock |
+----+----------+----------------+
| 111 | * | * | 50 |
| 222 | * | * | 50 |
| 333 | * | * | 50 |
| 444 | * | * | 50 |
| 555 | * | * | 50 |
| 666 | * | * | 50 |
+----+-------+-----+----+--------+
data_pinjam table
+---------------+----------------------------+
| no_transaksi | kode_buku | * | jumlah |
+---------------+-------------+----+---------+
| 1 | 111 | * | 3 |
| 1 | 222 | * | 2 |
| 1 | 333 | * | 4 |
+---------------+-------------+----+---------+
data_kembali table
+---------------+-----+----+---------+
| no_transaksi | * | * | status |
+---------------+-----+----+---------+
| 1 | * | * | back |
+---------------+-----+----+---------+
From my Tables, I create a function and trigger on table data_kembali. While Insert query to table data_kembali, function will make action to sum jumlah on table data_pinjam where no_transaksi in table data_kembali same with data_pinjam, and will update stock in table data_buku. Each rows with same kode_buku values. Stock + Jumlah.
I have create function
CREATE OR REPLACE FUNCTION kembali()
RETURNS TRIGGER AS
$BODY$
DECLARE
CURRENT_STOK INT4;
r data_pinjam%ROWTYPE;
BEGIN
FOR r IN
SELECT *
FROM data_pinjam p
WHERE p.no_transaksi = new.no_transaksi
LOOP
select CURRENT_STOK stock from data_buku where kode_buku = r.kode_buku;
CURRENT_STOK = CURRENT_STOK + r.jumlah;
UPDATE data_buku SET STOCK = CURRENT_STOK WHERE kode_buku = r.kode_buku;
END LOOP;
UPDATE data_pinjam SET status = 'kembali' WHERE no_transaksi = new.no_transaksi;
update data_transaksi set status = 'kembali' where no_transaksi = new.no_transaksi;
RETURN NEW;
END;
$BODY$
LANGUAGE PLPGSQL VOLATILE
COST 100;
but while running, get output
ERROR: query has no destination for result data
SQL state: 42601
Hint: If you want to discard the results of a SELECT, use PERFORM instead.
Context: PL/pgSQL function kembali() line 13 at SQL statement
Can someone advise me regarding this trigger and function for update with loop?

Answer 1

The SELECT statement in the cursor loop requires a destination for the selected value:
SELECT stock
INTO CURRENT_STOK
FROM data_buku
WHERE kode_buku = r.kode_buku;
The variable CURRENT_STOK would also need to be declared (before the BEGIN).
But perhaps the intent of the cursor for loop is this?
UPDATE data_buku
SET STOCK = STOCK + r.jumlah
WHERE kode_buku = r.kode_buku;