mysql sum showing wrong answer for decimal type

sales_id = 12

product_rate = 2090.00

making_charge = 83.60

handling_charge = 0.00

all of them are decimal(10,2) type. My query is

SELECT SUM( making_charge + product_rate + handling_charge ) AS tax_tourn
FROM  `sales_details` 
WHERE  `sales_id` =12

the result i am getting is

tax_tourn = 10784.40

which is not right

2090.00+83.60+0.00=2173.60

why is mysql showing wrong answer?

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Details Newly Added

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I have two tables sales and sales_details . sales_id is the primary key of sales and sales_details_id is the primary key for sales_details.

sales_id is the foreign key in sales_开发者_高级运维details.

There can multiple rows in sales_details with same sales_id.

But in our case (sales_id =12) there is only one row.

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It sounds like you have more than one row where sales_id =12.

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If this code returns more than 1 as the answer you have more than 1 row matching your where clause.

SELECT COUNT(*)
FROM  `sales_details` 
WHERE  `sales_id` =12