Postgres 9.6

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CREATE TABLE test (
    document_key text,
    amount_net int,
    classid int
);

INSERT INTO test VALUES
('201808', 10, 4021),
('201808', 10, 4021),
('201808', 15, 4023),
('201808', 13, 4023),
('201807', 20, 4021),
('201807', 15, 4021);

SELECT * FROM test;
6 rows affected



document_key
amount_net
classid


201808
10
4021

201808
10
4021

201808
15
4023

201808
13
4023

201807
20
4021

201807
15
4021


SELECT 
    document_key, 
    COALESCE(SUM(amount_net) FILTER (WHERE classid = 4021), 0) as "4021", 
    COALESCE(SUM(amount_net) FILTER (WHERE classid = 4023), 0) as "4023"
FROM test 
GROUP BY document_key
ORDER BY document_key


document_key
4021
4023


201807
35
0

201808
20
28


SELECT 
    document_key, 
    SUM(CASE WHEN classid = 4021 THEN amount_net ELSE 0 END) as "4021", 
    SUM(CASE WHEN classid = 4023 THEN amount_net ELSE 0 END) as "4023"
FROM test 
GROUP BY document_key
ORDER BY document_key


document_key
4021
4023


201807
35
0

201808
20
28

document_key	amount_net	classid
201808	10	4021
201808	10	4021
201808	15	4023
201808	13	4023
201807	20	4021
201807	15	4021