Question

I have the following tables

Groups
- Id
- Name

People
- Id
- GroupId
- EntryDate (DateTime)

Now I want to retrieve all groups sorted by group’s People entrydate that’s the latest.

//pseudo code

select * from groups order by (select top 1 from people order by entrydate desc)

Would it be better to add a LatestEntryDate field to Groups and update that when a person is added? since this will be a common query.

Answer 1

SELECT g.Id, g.Name, MAX(p.EntryDate) AS LatestPeopleEntryDate
FROM Groups g
    INNER JOIN People p
        ON p.GroupId = g.Id
GROUP BY g.Id, g.Name
ORDER BY MAX(p.EntryDate)

No, I wouldn’t add a LatestEntryDate field to the Groups table. This is easily calculated.

As @danihp pointed out, you may want to use a left join here. The above query will require that a value exists in the People table. What you choose to use depends on your specific requirements.

SELECT g.Id, g.Name, MAX(p.EntryDate) AS LatestPeopleEntryDate
FROM Groups g
    LEFT JOIN People p
        ON p.GroupId = g.Id
GROUP BY g.Id, g.Name
ORDER BY MAX(p.EntryDate)

UPDATE

To answer your question about doing this in Linq, I think it would be something like this:

var result = groups.Select(g => 
    new {
            g.Id, 
            g.Name, 
            LatestPeopleEntryDate = g.People.Max(p => p.EntryDate)
        });