I know it's an old topic, but I guess that it never loses actuality.
I'm developing something like that right now.
Here is my approach.
I use a server setting with a MySQL, Apache, PHP, and Zend Framework 2 as application framework, but it should work as well with any other settings.
Here is a simple implementation guide, you can evolve it yourself further from this.
You would need to implement your own query language interpreter, because the effective SQL would be too complicated.
Example:
select id, password from user where email_address = "xyz@xyz.com"
The physical database layout:
Table 'specs': (should be cached in your data access layer)
- id: int
- parent_id: int
- name: varchar(255)
Table 'items':
- id: int
- parent_id: int
- spec_id: int
- data: varchar(20000)
Contents of table 'specs':
- 1, 0, 'user'
- 2, 1, 'email_address'
- 3, 1, 'password'
Contents of table 'items':
- 1, 0, 1, ''
- 2, 1, 2, 'xyz@xyz.com'
- 3, 1, 3, 'my password'
The translation of the example in our own query language:
select id, password from user where email_address = "xyz@xyz.com"
to standard SQL would look like this:
select
parent_id, -- user id
data -- password
from
items
where
spec_id = 3 -- make sure this is a 'password' item
and
parent_id in
( -- get the 'user' item to which this 'password' item belongs
select
id
from
items
where
spec_id = 1 -- make sure this is a 'user' item
and
id in
( -- fetch all item id's with the desired 'email_address' child item
select
parent_id -- id of the parent item of the 'email_address' item
from
items
where
spec_id = 2 -- make sure this is a 'email_address' item
and
data = "xyz@xyz.com" -- with the desired data value
)
)
You will need to have the specs table cached in an associative array or hashtable or something similar to get the spec_id's from the spec names. Otherwise you would need to insert some more SQL overhead to get the spec_id's from the names, like in this snippet:
Bad example, don't use this, avoid this, cache the specs table instead!
select
parent_id,
data
from
items
where
spec_id = (select id from specs where name = "password")
and
parent_id in (
select
id
from
items
where
spec_id = (select id from specs where name = "user")
and
id in (
select
parent_id
from
items
where
spec_id = (select id from specs where name = "email_address")
and
data = "xyz@xyz.com"
)
)
I hope you get the idea and can determine for yourself whether that approach is feasible for you.
Enjoy! :-)
