Question

I want to record user states and then be able to report historically based on the record of changes we’ve kept. I’m trying to do this in SQL (using PostgreSQL) and I have a proposed structure for recording user changes like the following.

CREATE TABLE users (   userid SERIAL NOT NULL PRIMARY KEY,    name VARCHAR(40),    status CHAR NOT NULL );  CREATE TABLE status_log (   logid SERIAL,    userid INTEGER NOT NULL REFERENCES users(userid),    status CHAR NOT NULL,    logcreated TIMESTAMP ); 

That’s my proposed table structure, based on the data.

For the status field ‘a’ represents an active user and ‘s’ represents a suspended user,

INSERT INTO status_log (userid, status, logcreated) VALUES (1, 's', '2008-01-01');  INSERT INTO status_log (userid, status, logcreated) VALUES (1, 'a', '2008-02-01');  

So this user was suspended on 1st Jan and active again on 1st of February.

If I wanted to get a suspended list of customers on 15th January 2008, then userid 1 should show up. If I get a suspended list of customers on 15th February 2008, then userid 1 should not show up.

1) Is this the best way to structure this data for this kind of query?

2) How do I query the data in either this structure or in your proposed modified structure so that I can simply have a date (say 15th January) and find a list of customers that had an active status on that date in SQL only? Is this a job for SQL?

Answer 1

This can be done, but would be a lot more efficient if you stored the end date of each log. With your model you have to do something like:

select l1.userid from status_log l1 where l1.status='s' and l1.logcreated = (select max(l2.logcreated)                      from status_log l2                      where l2.userid = l1.userid                      and   l2.logcreated <= date '2008-02-15'                     ); 

With the additional column it woud be more like:

select userid from status_log where status='s' and logcreated <= date '2008-02-15' and logsuperseded >= date '2008-02-15'; 

(Apologies for any syntax errors, I don’t know Postgresql.)

To address some further issues raised by Phil:

A user might get moved from active, to suspended, to cancelled, to active again. This is a simplified version, in reality, there are even more states and people can be moved directly from one state to another.

This would appear in the table like this:

userid  from       to         status FRED    2008-01-01 2008-01-31 s FRED    2008-02-01 2008-02-07 c FRED    2008-02-08            a 

I used a null for the ‘to’ date of the current record. I could have used a future date like 2999-12-31 but null is preferable in some ways.

Additionally, there would be no ‘end date’ for the current status either, so I think this slightly breaks your query?

Yes, my query would have to be re-written as

select userid from status_log where status='s' and logcreated <= date '2008-02-15' and (logsuperseded is null or logsuperseded >= date '2008-02-15'); 

A downside of this design is that whenever the user’s status changes you have to end date their current status_log as well as create a new one. However, that isn’t difficult, and I think the query advantage probably outweighs this.