conceptual SQL - price per piece and price per weight [closed] - sql

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This is a rather conceptual question:
I am working on a database with tables "product-information" and "buying transactions" - now the product-information table so far features "price" and the buying-transactions table features "amount".
But, some products are supposed to be paid per piece, and some have a price per weight.
Now I am unsure how to go about this without allowing decimal values for amounts.
Should I have the products have a flag if they are to be processed as price per weight and just do further calculations in the surrounding program? That seems to be rather impractical, since it makes aggregation in queries rather impossible. Or should I allow decimals but prohibit them in the user interface for things to be bought by piece, again requiring a flag?
What is the most sensible approach here?
Basically imagine a database containing receipts from groceries shopping and the appropriate information for each product. The user would insert the contents of a cart and the sum total would be calculated and spit out by the program, as well as the calculated price for each article to be paid per weight.
I'm sorry for the stupidity of the question.

Answer 1

Here's what we do for LedgerSMB, and I think the solution works relatively well.
Items are all priced per "unit." Items have a price per unit and a unit descriptor (human readable).
Items sold per piece have a unit of "piece". Items sold per weight have a unit like "kg", "oz", "g" or "T"
Price and quantity sold are both Numeric types without precision specified (so in PostgreSQL at least you have no precision limits).
Our table structure looks something like this (simplified for this question)
CREATE TABLE parts (
id SERIAL PRIMARY KEY,
sku VARCHAR NOT NULL,
unit BARCHAR(5) NOT NULL,
sell_price NUMERIC,
last_cost NUMERIC,
description TEXT,
obsolete BOOL
);
CREATE UNIQUE INDEX parts_sku_active_idx ON parts(sky) WHERE obsolete IS NOT TRUE;