Question

(I wish I could have come up with a more descriptive title… suggest one or edit this post if you can name the type of query I’m asking about)

Database: SQL Server 2000

Sample Data (assume 500,000 rows):

Name   Candy       PreferenceFactor
Jim    Chocolate   1.0
Brad   Lemon Drop   .9
Brad   Chocolate    .1
Chris  Chocolate    .5
Chris  Candy Cane   .5
499,995 more rows...

Note that the number of rows with a given ‘Name’ is unbounded.

Desired Query Results:

Jim    Chocolate   1.0
Brad   Lemon Drop   .9
Chris  Chocolate    .5
~250,000 more rows...

(Since Chris has equal preference for Candy Cane and Chocolate, a consistent result is adequate).

Question:
How do I Select Name, Candy from data where each resulting row contains a unique Name such that the Candy selected has the highest PreferenceFactor for each Name. (speedy efficient answers preferred).

What indexes are required on the table? Does it make a difference if Name and Candy are integer indexes into another table (aside from requiring some joins)?

Answer 1

select c.Name, max(c.Candy) as Candy, max(c.PreferenceFactor) as PreferenceFactor
from Candy c
inner join (
    select Name, max(PreferenceFactor) as MaxPreferenceFactor
    from Candy
    group by Name
) cm on c.Name = cm.Name and c.PreferenceFactor = cm.MaxPreferenceFactor
group by c.Name
order by PreferenceFactor desc, Name