MariaDB 10.5

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CREATE TABLE receiptitem (retailprice decimal(9, 2),
                          vat decimal(5, 4))
INSERT INTO receiptitem
            (retailprice,
             vat)
            VALUES (120,
                    .2);
CREATE VIEW v_receiptsumbyvat_erroneous
AS
SELECT vat,
       sum(RetailPrice) / (1 + vat) sum
       FROM receiptitem
       GROUP BY vat;
CREATE VIEW v_receiptsumbyvat
AS
SELECT vat,
       sum / (vat + 1) sum
       FROM (SELECT vat,
                    sum(retailprice) sum
                    FROM receiptitem
             GROUP BY vat) x;
SELECT vat,
       sum(RetailPrice) / (1 + vat) sum
       FROM receiptitem
       GROUP BY vat;


vat
sum


0.2000
100.000000


SELECT vat,
       sum / (vat + 1) sum
       FROM (SELECT vat,
                    sum(retailprice) sum
                    FROM receiptitem
             GROUP BY vat) x;


vat
sum


0.2000
100.000000


SELECT *
       FROM v_receiptsumbyvat_erroneous;


vat
sum


0.2000
120.200000


SELECT *
       FROM v_receiptsumbyvat;


vat
sum


0.2000
100.000000