Each system (computer, network device, printer, et al) has an entry in the "system" table.
Each system (from the "system" table) has a "system_id" column.
This value is unique - it's an auto incrementing id.
A system is determined to be unique by a the table below.
A system is audited and the result submitted to the server.
The first table processed is the "system" table.
The "system_id" is determined and passed (along with the other details) to each other section (table).
Every table has two timestamp columns, "first_timestamp" and "timestamp".
The "first_timestamp" value is populated whenever an insert occurs - hence this value reflects the first time an item was reported in the audit script.
The "timestamp" value is inserted when an item is first seen, or updated when an item is seen in subsequent audit script(s).
There is a "oa_audit_log" table that contains details of each time an audit is submitted (including timestamp).
So, for an example - "hard_drive".
The "system_id" is retrieved, along with the timestamp of the previous audit submission and the "status" column.
For each entry in the hard_drive audit result, the database is queried.
It checks for hard drive model, serial, index and size.
These values vary according to the item being processed - see the relevent model PHP pages (in code_igniter/application/models/).
If it gets a match on the above values, combined with the system_id, the current timestamp OR the previous timestamp, and a status of "production", then an existing entry exists for this piece of equipment.
In the case of hard drives, it simply updates the timestamp to reflect the current audit timestamp.
If it does not get a match, it does an insert of the relevant details.
So, we can determine if something is currently installed - the timestamps match (on the system table and the relevant item table).
We can determine when something was detected - the "first_timestamp".
We can determine if something was installed after the initial audit - first timestamps will be different.
We can determine if something is not currently installed, but previously was - the timestamp on the item is less than the timestamp on the system.
We can determine the last time we detected an item - the timestamp on the item, when the timestamp is less than the current system timestamp.
At any given point, we can determine what was on a system - by using the oa_audit_log table and selecting the relevant components based on timestamps.
So, that's how we determine what's on or has been on a system.