Postgres 12

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CREATE TABLE t(Account INT, Product_A INT, Product_B INT, Product_C INT, Product_D INT );

CREATE TABLE


INSERT INTO t VALUES
(101, 1000, 2000, 3000, 4000),
(102, 2000, 1000, 0, 1500),
(103, 2000, 1000, 0, 1780),
(104, 2000, 1000, 2000, 100);

INSERT 0 4


SELECT account, UPPER(SPLIT_PART((js).key, '_', 2)) AS Product, (js).value AS Qty 
  FROM( SELECT rj->>'account' AS Account, JSON_EACH_TEXT(rj) AS js
          FROM ( SELECT ROW_TO_JSON(t) AS rj FROM t) AS q1 ) AS q2
 WHERE (js).key != 'account';


account
product
qty


101
A
1000

101
B
2000

101
C
3000

101
D
4000

102
A
2000

102
B
1000

102
C
0

102
D
1500

103
A
2000

103
B
1000

103
C
0

103
D
1780

104
A
2000

104
B
1000

104
C
2000

104
D
100



SELECT 16

account	product	qty
101	A	1000
101	B	2000
101	C	3000
101	D	4000
102	A	2000
102	B	1000
102	C	0
102	D	1500
103	A	2000
103	B	1000
103	C	0
103	D	1780
104	A	2000
104	B	1000
104	C	2000
104	D	100