How to store data with dynamic number of attributes in a database

Question

I have a number of different objects with a varying number of attributes. Until now I have saved the data in XML files which easily allow for an ever changing number of attr

Answer 1

I used to implement this scheme:

t_class (id RAW(16), parent RAW(16)) -- holds class hierachy.
t_property (class RAW(16), property VARCHAR) -- holds class members.
t_declaration (id RAW(16), class RAW(16)) -- hold GUIDs and types of all class instances
t_instance (id RAW(16), class RAW(16), property VARCHAR2(100), textvalue VARCHAR2(200), intvalue INT, doublevalue DOUBLE, datevalue DATE) -- holds 'common' properties

t_class1 (id RAW(16), amount DOUBLE, source RAW(16), destination RAW(16)) -- holds 'fast' properties for class1.
t_class2 (id RAW(16), comment VARCHAR2(200)) -- holds 'fast' properties for class2
--- etc.

RAW(16) is where Oracle holds GUIDs

If you want to select all properties for an object, you issue:

SELECT  i.*
FROM    (
        SELECT  id 
        FROM    t_class
        START WITH
                id = (SELECT class FROM t_declaration WHERE id = :object_id)
        CONNECT BY
                parent = PRIOR id
        ) c
JOIN    property p
ON      p.class = c.id
LEFT JOIN
        t_instance i
ON      i.id = :object_id
        AND i.class = p.class
        AND i.property = p.property

t_property hold stuff you normally don't search on (like, text descriptions etc.)

Fast properties are in fact normal tables you have in the database, to make the queries efficient. They hold values only for the instances of a certain class or its descendants. This is to avoid extra joins.

You don't have to use fast tables and limit all your data to these four tables.