Get top 1 row of each group

Question

I have a table which I want to get the latest entry for each group. Here\'s the table:

DocumentStatusLogs Table

|ID| DocumentID | Status         


        

Answer 1

In scenarios where you want to avoid using row_count(), you can also use a left join:

select ds.DocumentID, ds.Status, ds.DateCreated 
from DocumentStatusLogs ds
left join DocumentStatusLogs filter 
    ON ds.DocumentID = filter.DocumentID
    -- Match any row that has another row that was created after it.
    AND ds.DateCreated < filter.DateCreated
-- then filter out any rows that matched 
where filter.DocumentID is null 

For the example schema, you could also use a "not in subquery", which generally compiles to the same output as the left join:

select ds.DocumentID, ds.Status, ds.DateCreated 
from DocumentStatusLogs ds
WHERE ds.ID NOT IN (
    SELECT filter.ID 
    FROM DocumentStatusLogs filter
    WHERE ds.DocumentID = filter.DocumentID
        AND ds.DateCreated < filter.DateCreated)

Note, the subquery pattern wouldn't work if the table didn't have at least one single-column unique key/constraint/index, in this case the primary key "Id".

Both of these queries tend to be more "expensive" than the row_count() query (as measured by Query Analyzer). However, you might encounter scenarios where they return results faster or enable other optimizations.