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Say I have a store that sells products that fall under various categories... and each category has associated properties... like a drill bit might have coating, diameter, helix angle, or whatever. The issue is that I'd like the user to be able to edit these properties. If I wasn't interested in having the user change the properties, and I was building the store for a certain set of categories, I'd have one table for drill bits, etc. Alternatively, I could just modify the schema online but that doesn't seem to be done very often (unless we're talking phpmyadmin or something), and plus that doesn't fit in well at all with the way models are coupled to tables.

In general, I'm interested in implementing a multi-table database structure with various datatypes (because diameter might be a decimal, coating would be a string/index into a table, etc), within mysql. Any idea how this might be done?

Evan
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6 Answers6

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If I understand correctly what you're asking, an, admittedly hacky, solution would be to have a products table that has to related tables, product_properties and product_properties_lookup (or some better name) where product_properties_lookup has an entry for every possible property a product can have and where product_properties contains the value of a property as a string with the ID of the property and the ID of the product. You could then coerce the property value into whatever type you wanted. Not ideal, but I'm not sure what else to do short of adding individual columns to the DB for property types.

baudtack
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Just use the database. It does all of this already. For free. And fast. How is having a table of products point to a table of properties with data types any different from a table with columns? It's not. Save if you use the DBs tables you get to use SQL to query it in all sorts of neat, and efficient ways compared to your own (crosstabs suck in SQL dbs).

Get a new product, make a new table. No big deal. Get a new property, alter the table. If you have 1M products in that table, yea, it may be a slow update (depends on the DB). Do you have 1M products? I don't think WalMart has 1M products.

Building Databases on top of Databases is a silly thing. Just use the one that's there. It is putty in your hands. Mold it to your whim.

Will Hartung
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  • I think Evan is wanting to do this on the fly. I wouldn't be comfortable myself issuing `alter table` statements based on users adding new properties. – baudtack Jul 22 '10 at 23:47
  • Why not? Databases do this all day long. An Alter table effectively locks the table. Big deal. I just altered a (non-trivial) Postgres table with 630,000 rows, and adding a column took 1s. Dropping the column took 66ms. How many items do you think are going to be in these tables? If it's a problem, do it at a low traffic point. Databases do this all really well so we don't have to. – Will Hartung Jul 23 '10 at 00:36
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Create a Property table first. This will contain all properties. It should have (at minimum) a Name column and a Type column ('string', 'boolean', 'decimal', etc.). Note: Primary keys are implied for all these tables.

Next, create a CategoryProperty table. Here you will be able to assign properties to a category. It should have these columns: CategoryID, PropertyID. Both foreign keys.

Then, create a Category table. This describes the categories. It should have a Name column and possibly some other columns like Description.

Then, create a ProductCategory table. Here, you will assign the categories for each product. It should have these columns: CategoryID, ProductID. Both foreign keys.

Next, create a PropertyValue table. Here, you will "instantiate" the properties and give them values. Columns include ProductID, PropertyID, and PropertyValue. The primary key can consist of ProductID and PropertyID.

Finally, create a Product table that just describes each product with columns like Name, Price, etc.

Note how for each relationship there is a separate table. If you only want one category for each product, you can do away with the ProductCategory table and just put a CategoryID field in the Product table. Similarly, if you want each property to belong to only one category, you can put a PropertyID column in the Category table and get rid of the CategoryProperty table.

Lastly, you will not be able to verify the data type for each property since each property has a different type (and they are rows, not columns). So just make the PropertyValue column a string and then perform your validation either as a trigger, or in your application, by checking the Type column of the Property table for that property.

dacris
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If you're using a recentish version of mysql (5.1.5 or greater) you can store your data as XML in the database. You can then query that data using thigns like this.

Suppose I have a table that contains some items and I have a widgetpack that contains numerous widgets. I can get my total number of widgets:

SELECT SUM( EXTRACTVALUE( infoxml, '/info/widget_count/text()' ) ) as widget_count 
  WHERE product_type="widgetpack"

assuming the table has an infoxml column and each widgetpacks infxml column contain XML that looks like this

<info>
  <widget_count>10</widget_count>
  <!-- Any other unstructured info can go in here too -->
</info>

DB purists will cringe at this, and it is kinda hacky. But often its easier to keep all your unstructured data in one place.

Michael Anderson
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  • I'm dealing with this scenario now on a project I'm on. The part I don't like is the fact that the only way I can upgrade the xml formatting is to use XSLT transformations... any suggestions on a better method to upgrade the data? – Richard B Jul 23 '10 at 02:43
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Have a look at this database schema on DatabaseAnswers.org:

http://www.databaseanswers.org/data_models/products_and_generic_characteristics/index.htm

Robert Harvey
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Maybe consider an Entity-Attribute-Value (EAV) approach (not for the whole model of course!).

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