Question

I have a database where I store objects. I have the following (simplified) schema

CREATE TABLE MyObjects (   UniqueIdentifier Id;   BigInt           GenerationId;   BigInt           Value;   Bit              DeleteAction; ) 

Each object has a unique identifier (‘Id’), and a (set of) property (‘Value’). Each time the value of the property for an object is changed, I enter a new row into this table with a new generation id (‘GenerationId’, which is monotonically increasing). If an object is deleted, then I record this fact by setting the ‘DeleteAction’ bit to true.

At any point in time (generation), I would like to retrieve the state of all of my active objects!

Here’s an example:

Id    GenerationId Value  DeleteAction 1        1          99       false 2        1          88       false 1        2          77       false 2        3          88       true 

Objects in generations are:

  1: 1 {99}, 2 {88}   2: 1 {77}, 2 {88}   3: 1 {77} 

The key is: how can I find out the row for each unique object who’s generation id is closest (but not exceeding) to a given generation id? I can then do a post-filter step to remove all rows where the DeleteAction field is true.

Answer 1

Here’s the working version:

SELECT MyObjects.Id,Value FROM Myobjects INNER JOIN  (         SELECT Id, max(GenerationId) as LastGen   FROM MyObjects   WHERE GenerationId <= @TargetGeneration   Group by Id ) T1 ON MyObjects.Id = T1.Id AND MyObjects.GenerationId = LastGen WHERE DeleteAction = 'False'