Question

I have a simple query:

Select         Count(p.Group_ID)
From           Player_Source P 
  Inner Join   Feature_Group_Xref X On P.Group_Id=X.Group_Id 
where          x.feature_name ='Try this site'

which spits out the current number of people in a specific test group at the current moment in time.

If I wanted to see what this number was, say, on 9/10/12 instead, could I add something in to the query to time phase this information as the database had it 2 days ago?

Answer 1

No. If you want to store historical information, you will need to incorporate that into your schema. For example, you might extend Feature_Group_Xref to add the columns Effective_Start_Timestamp and Effective_End_Timestamp; to find which groups currently have a given feature, you would write AND Effective_End_Timestamp > CURRENT_TIMESTAMP() (or AND Effective_End_Timestamp IS NULL, depending how you want to define the column), but to find which groups had a given feature at a specific time, you would write AND ... BETWEEN Effective_Start_Timestamp AND Effective_End_Timestamp (or AND Effective_Start_Timestamp < ... AND (Effective_End_Timestamp > ... OR Effective_End_Timestamp IS NULL)).

Wikipedia has a good article on various schema designs that people use to tackle this sort of problem: see http://en.wikipedia.org/wiki/Slowly_changing_dimension.