Question

I have a table with the following columns:

ConnectionId INT - primary key, identity,
ServerId INT, 
ClientId INT, 
Time DATETIME, 
ConnectionType INT

How do I define an optimal view which returns all combinations of ServerId and ClientId and time and type of LAST connection for each combination?

Answer 1

Assuming you are using a database that supports ranking functions (such as SQL Server 2005+, Oracle, Postgres, DB2, Teradata . . .), you can do this as follows:

create view as
    select ServerId, ClientId, time, type
    from (select t.*,
                 row_number() over (partition by ServerId, ClientId order by time desc) as seqnum
          from t
         ) t
    where seqnum = 1

I don’t know what you mean by “optimal view”. If you want this to work better, create a three-part index on ServerId, ClientId, and time.