architecture - How to model multiple "uses" (e.g. weapon) for usable-inventory/object/items (e.g. katana) within a relational database

So I'm working on expanding the uses of items over at www.ninjawars.net, and I'm not exactly sure how to represent them flexibly in the relational database that we use.

I may be barking up the wrong tree, so feel free to make suggestions in other directions, but currently I'm thinking that each item should have relational "tags".

For example, a Katana is currently a row in the "items" database. To make it into a weapon, and a holdable thing, I was thinking that I would have a database of "traits", and a item_traits table that simply linked between them.

// Objects and their basic data

item_id | item
1 | Naginata



// Things that objects can do

trait_id | trait
1 | weapon
2 | holdable

// How those objects do those things, e.g. powerfully, weakly, while on fire

_item_id | _trait_id | item_trait_data
1 | 1 | damage: 5, damage_type: sharp, whatever, etc

I'm not really sure how to model the extra data that results (e.g. the damage that a sword will do, the damage_type, etc).

I'm also not especially happy that the whole of an item would be stored in more than one place, e.g. in order to create a copy of an item with a different name, like a "short sword", I would have to copy from multiple tables to create the duplicate item.

Is there a better way to lay this stuff out that I'm missing?

Edit: I should just note that I've already got a postgresql database in use on the site, which is why I want to use it for my data storage.

Edit: I've added an answer for the implementation that I'm currently looking at.

Answer

Here's what I'm now considering:

Since every "trait" essentially requires changes in the code anyway, so I've decided to just keep the traits (and any default data they require) in the code itself (at least for now).

E.g. $traits = array('holdable'=>1, 'weapon'=>1, 'sword'=>array('min_dam'=>1, 'max_dam'=>500));

Then items get a "trait_data" field in the database that will use the json_encode() function to get stored in the JSON format.

item_id | item_name | item_identity | traits
1 | Katana | katana | "{"holdable":1,"weapon":1,"sword":{"min_dam":1,"max_dam":1234,"0":{"damage_type":"fire","min_dam":5,"max_dam":50,"type":"thrown","bob":{},"ids":[1,2,3,4,5,6,7]}}}"

Plan is that items will inherit all the default stats from their parent traits, and will only have over-ride traits that specify differences from what the traits set as defaults.

The downside of the traits field is that while I could edit the json part by hand... ...it won't be really easy or safe to do so with data that's in the database.