How to get the last record per group in SQL

Question

I am facing a rather interesting problem. I have a table with the following structure:

CREATE TABLE [dbo].[Event]
(
    Id int IDENTITY(1,1) NOT NULL,
    ApplicationId nvarchar(32) NOT NULL,
    Name nvarchar(128) NOT NULL,
    Description nvarchar(256) NULL,
    Date nvarchar(16) NOT NULL,
    Time nvarchar(16) NOT NULL,
    EventType nvarchar(16) NOT NULL,
    CONSTRAINT Event_PK PRIMARY KEY CLUSTERED ( Id ) WITH (
        PAD_INDEX = OFF, 
        STATISTICS_NORECOMPUTE = OFF, 
        IGNORE_DUP_KEY = OFF, 
        ALLOW_ROW_LOCKS = ON, 
        ALLOW_PAGE_LOCKS  = ON
    )
)

So the issue is that I have to display this data in a grid. There are two requirements. The first one is to display all events regardless of what application threw them. This is simple - a select statement will do the job very easily.

The second requirement is to be able to group events by Application. In other words display all events in a way that if the ApplicationId is repeated more than once, grab only the last entry for every application. The primary key of the Event (Id) at this point is no longer needed in this query/view.

You may also notice that the Event Date and Time are in string format. This is ok because they follow the standard date time formats: mm/dd/yyyy and hh:mm:ss. I can pull those as follows:

Convert( DateTime, (Date + ' ' +  Time)) AS 'TimeStamp'

My issue is that if I use AGGREGATE functions on the rest of the columns I don't know how would they behave:

SELECT
    ApplicationId,
    MAX(Name),
    MAX(Description),
    MAX( CONVERT(DateTime, (Date + ' ' + Time))) AS 'TimeStamp',
    MAX( EventType )
FROM
    Event
GROUP BY
    ApplicationId

The reason I am hesitant to do so is because a function such as MAX will return the largest value for a given column from a (sub)set of records. It does not necessary pull the last record!

Any ideas on how to select only the last record on a per application basis?

Answer 1

You can use a ranking function and a common table expression.

WITH e AS
(
     SELECT *,
         ROW_NUMBER() OVER
         (
             PARTITION BY ApplicationId
             ORDER BY CONVERT(datetime, [Date], 101) DESC, [Time] DESC
         ) AS Recency
     FROM [Event]
)
SELECT *
FROM e
WHERE Recency = 1