Reducing round-trips to database - asp.net

Question

I'm using ASP.NET Web Forms and trying to load data from SQL server. Here's the pseudo-code on how I do it:
connect1 = connect("database")
categories = connect.query("select * from category")
loop categories as category
print category
connect2 = connect("database")
subCategories = connect2.query("select * from subCategory where category = #0", category)
loop subCategories as subCategory
print subCategory
connect3 = connect("database")
items = connect3.query("select * from item where subCategory = #0", subCategory)
loop items as item
print item
end loop 'items
connect3.close
end loop 'subcategories
connect2.close
end loop 'categories
connect1.close
As you can see, there are lots of round-trips going on in my script, this is fine when I only have few records but when dealing with hundreds or more, this takes forever to display the data.
What can I do to reduce the number of round-trips? I thought of getting all the data at once from the database then categorize them in the application side but is that possible?

Answer 1

Why don't you get all data you need by one query with joins and then filter in on a client-side; Or other way you can do it (it there's not too much data) is getting data as xml, deserialize it to ienumerable an iterate through.
As i see it you do
categories = connect.query("select * from category");
so all you need is:
whole_data = connect.query("select * from category c inner join subCategory sc on c.id = sc.id inner join item i on i.id = si.id") /*or some kind of*/
/*let me think that whole_data is a list of objects, not a dataset*/
categories = whole_data.Distinct(d => d.category);
subCategories = whole_data.Distinct(d => d.subCategories);
/*and then do your loops*/
c# code for manual mapping might be like that:
using (var connection = new SqlConnection(connString))
{
connection.Open();
var command = connection.CreateCommand();
command.CommandText = "select * from ...";
var reader = command.ExecuteReader();
while (reader.Read())
{
var a = reader.GetInt32(0);
var b = reader.GetString(1);
/*so basically you read all fields that you get from DB and build object here, then you add it to List or any other kind of collection*/
}
}

Answer 2

Depending upon the latency to your database, making the connections, even if pooled, can take a long time. To avoid that over head, make all your connections outside of the loops. That is, don't nest the connections. Instead, structure it like this:
Connect1 = connect("database")
Connect2 = connect("database")
Connect3 = connect("database")
sql 1 nest
sql 2 nest
sql 3 nest
end nest
end nest
end nest
close connections.
If you have 10 entries per loop, and a connection takes 10 mSec, you will spend 10 x 10 x 10 = 1000 mS just doing connections. Take them outside the nest, then you spend only 30 mS making connections. Close your datareader at the completion of each nest so the connection can be reused.
Of course, for the example you showed, doing a single query is the best solution.
But, if your query is selective and you need to perform some business logic that cannot be combined in the query, then always move your connections outside of the loop.