MySQL 8.0

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select version();


version()


8.0.16


create table products (id int primary key, name varchar(30), cost float)
create table product_tax (product_id int, tax_id int, primary key(product_id,tax_id))
create table tax (id int primary key, taxname varchar(15), rate float)
insert into products values (1,"bunny", 20), (2,"truck", 30)
insert into tax values (1, 'state', 0.1), (2, 'feral', 0.05)
insert into product_tax values (1, 1) , (1,2)
select products.id, products.name, ROUND((1 + IFNULL(SUM(rate),0)) * cost,2) FROM products LEFT JOIN product_tax ON products.id = product_tax.product_id LEFT JOIN tax ON product_tax.tax_id = tax.id GROUP BY products.id


id
name
ROUND((1 + IFNULL(SUM(rate),0)) * cost,2)


1
bunny
23.00

2
truck
30.00