Question

Lets say you have a relational DB table like INVENTORY_ITEM. It’s generic in the sense that anything that’s in inventory needs a record here. Now lets say there are tons of different types of inventory and each different type might have unique fields that they want to keep track of (e.g. forks might track the number of tines, but refrigerators wouldn’t have a use for that field). These fields must be user-definable per category type.

There are many ways to solve this:

1. Use ALTER TABLE statements to actually add nullable columns on the fly (yuk)
2. Have two tables with a one-to-one mapping, INVENTORY_ITEM, and INVENTORY_ITEM_USER, and use ALTER TABLE statements to add and remove nullable columns from the latter table on the fly (a bit nicer).
3. Add a CUSTOM_PROPERTY table, and a CUSTOM_PROPERTY_VALUE table, and add/remove rows in CUSTOM_PROPERTY when the user adds and removes rows, and store the values in the latter table. This is nice and generic, but the performance would suffer. If you had an average of 20 values per item, the number of rows in CUSTOM_PROPERTY_VALUE goes up at 20 times the rate, and you still need to include columns in CUSTOM_PROPERTY_VALUE for every different data type that you might want to store.
4. Have one big varchar(MAX) field on INVENTORY_ITEM to store custom properties as XML.
5. I guess you could have individual tables for each category type that hangs off the INVENTORY_ITEM table, and these get created/destroyed on the fly when the user creates inventory types, and the columns get updated when they add/remove properties to those types. Seems messy though.

Is there a best-practice for this? It seems to me that option 4 is clean, but doesn’t allow you to easily search by the metadata. I’ve used a variant of 3 before, but only on a table that had a really small number of rows, so performance wasn’t an issue. It always seemed to me that 2 was a good idea, but it doesn’t fit well with auto-generated entity frameworks, so you’d have to exclude the custom properties table from the entity generation and just write your own custom data access code to handle it.

Am I missing any alternatives? Is there a way for SQL server to “look into” XML data in a column so it could actually do stuff with option 4 now?

Answer 1

I am using the xml type column for this kind of situations…

http://msdn.microsoft.com/en-us/library/ms189887.aspx

Before xml we had to use the option 3. Which in my point of view is still a good way to do it. Espacialy if you have a Data Access Layer that is able to handle the type conversion properly for you. We stored everything as string values and defined a column that held the orignial data type for the conversion.

Options 1 and 2 are a no-go. Don’t change the database schema in production on the fly.

Option 5 could be done in a separate database… But still no control over the schema and the user would need the rights to create tables etc.