Query to get all revisions of an object graph

Question

I'm implementing an audit log on a database, so everything has a CreatedAt and a RemovedAt column. Now I want to be able to list all revisions of an object graph but the best way I can think of for this is to use unions. I need to get every unique CreatedAt and RemovedAt id.

If I'm getting a list of countries with provinces the union looks like this:

SELECT c.CreatedAt AS RevisionId from Countries as c where localId=@Country
UNION
SELECT p.CreatedAt AS RevisionId from Provinces as p 
INNER JOIN Countries as c ON p.CountryId=c.LocalId AND c.LocalId = @Country
UNION
SELECT c.RemovedAt AS RevisionId from Countries as c where localId=@Country
UNION
SELECT p.RemovedAt AS RevisionId from Provinces as p 
INNER JOIN Countries as c ON p.CountryId=c.LocalId AND c.LocalId = @Country

For more complicated queries this could get quite complicated and possibly perform very poorly so I wanted to see if anyone could think of a better approach. This is in MSSQL Server.

I need them all in a single list because this is being used in a from clause and the real data comes from joining on this.

Answer 1

You have most likely already implemented your solution, but to address a few issues; I would suggest considering Aleris's solution, or some derivative thereof.

In your tables, you have a "removed at" field -- well, if that field were active (populated), technically the data shouldn't be there -- or perhaps your implementation has it flagged for deletion, which will break the logging once it is removed.

What happens when you have multiple updates during a reporting period -- the previous log entries would be overwritten.

Having a separate log allows for archival of the log information and allows you to set a different log analysis cycle from your update/edit cycles.

Add whatever "linking" fields required to enable you to get back to your original source data OR make the descriptions sufficiently verbose.

The fields contained in your log are up to you but Aleris's solution is direct. I may create an action table and change the field type from varchar to int, as a link into the action table -- forcing the developers to some standardized actions.

Hope it helps.

Answer 2

An alternative would be to create an audit log that might look like this:

AuditLog table
    EntityName varchar(2000),
    Action varchar(255),
    EntityId int,
    OccuranceDate datetime

where EntityName is the name of the table (eg: Contries, Provinces), the Action is the audit action (eg: Created, Removed etc) and the EntityId is the primary key of the modified row in the original table.

The table would need to be kept synchronized on each action performed to the tables. There are a couple of ways to do this:

1) Make triggers on each table that will add rows to AuditTable
2) From your application add rows in AuditTable each time a change is made to the repectivetables

Using this solution is very simple to get a list of logs in audit.

If you need to get columns from original table is also possible using joins like this:

select *
from 
    Contries C 
    join AuditLog L on C.Id = L.EntityId and EntityName = 'Contries'

Answer 3

You could probably do it with a cross join and coalesce, but the union is probably still better from a performance standpoint. You can try testing each though.

SELECT
     COALESCE(C.CreatedAt, P.CreatedAt)
FROM
     dbo.Countries C
FULL OUTER JOIN dbo.Provinces P ON
     1 = 0
WHERE
     C.LocalID = @Country OR
     P.LocalID = @Country