afan,
>For the same project (below) I have problem with building table for
product prices.
>In "regular" online store, price is usually part of the products table.
>But, I need a solution for multiple prices. E.g.
>QTY - 25 50 100 200
>Price - $1.59 $1.39 $1.19 $0.99
>Also, if product is On Sale I need to be shown both prices: regular
and sale price
>QTY - 25 50 100 200
>Price - $1.59 $1.39 $1.19 $0.99
>Sale - $0.99 $0.99 $0.99 $0.99
First two footnotes to the excellent advice offered by Rhino & Shawn on
your categories, products & products_categories tables:
1. It will be best to type the primary & foreign keys identically--all
unsigned, or all not.
2. To avoid rounding errors, use DECIMAL rather than FLOAT for money
columns.
Before you model extended price computations, you have to ask & answer
crucial questions:
(i) do you know in advance all the kinds of price extensions that can
come up?
(ii) do you want the price rules to be (a) in the database or (b) in the
app?
(iii) if the answer to (ii) is (a), do you want the rules in stored
procedures, or in tables which application code must parse?
(iv) does the app need to track price history (eg so it can recreate a
price computation from six months ago)?
Suppose the answers are those that most conventionally apply: only qty
and sale will ever come up, the rules will be in the app, and you can
leave history to the backups. Then you can take a very simple,
semi-normalised approach (leaving out some details):
CREATE TABLE extended_prices (
epid INT AUTO_INCREMENT PRIMARY KEY,
product_id INT NOT NULL,
qty_up_to SMALLINT NOT NULL,
price_per DECIMAL(10,2) NOT NULL,
price_per_sale DECIMAL(10,2) NULL
);
which permits you to enter whatever (qty cutoffs, price, sale) combos
are desired for any desired products, and find them for any product with
a very simple query. There is a risk, though: in six months the client
may find that new price extensions are needed, and/or that she needs
history after all.
Now, add the wrinkles that other possible, but presently unidentified
price extensions (eg 'special promotions', 'coupons', &c) will be
required, and that history must be tracked. Now you need at least, again
normalising only partly...
CREATE TABLE pricemodtypes (
pricemodtype_id INT AUTO_INCREMENT PRIMARY KEY,
name CHAR(20)
)
CREATE TABLE extended_prices (
epid INT AUTO_INCREMENT PRIMARY KEY,
product_id INT NOT NULL,
pricemodtype_id INT NOT NULL,
qty_up_to SMALLINT NOT NULL,
begindate DATE NOT NULL,
enddate DATE NOT NULL,
price_per DECIMAL(10,2) NOT NULL,
price_per_mod DECIMAL(10,2) NULL
);
the query to retrieve all the prices for a product is more complicated
but still straightforward.
Of course wrinkles multiply as if conjured by a Sorcerer's Apprentice.
Perhaps we should pause here for a breath. Is this the info you need?
PB
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