Question

How do I retrieve relational data from multiple tables at once

Asked by: TinoNL

I am a VB.Net developer  and I would like to learn how to retrieve relational data from an SQL database, where the relation data is immediately put into related business objects.

I have created an example VS2008 project, that shows how I currently fill my Business Objects.  My example has a Company table and a Order table, which have a 1:N relation. http://www.spanishharlemimport.nl/teachme.rar

In my example, all Companies are retrieved first, and second the orders per company. For a small query, like the example the performance is still fine, but Im on a project that has to retrieve > 100.000 of related records.

I believe that LINQ might offer a solution for my problem, in which whole related tables can be retrieved to Business Objects in 1 run. Does anyone knows a good tutorial, or could someone modify my project, to show how it could be more efficient?

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Asked On
2009-10-07 at 07:30:04ID24792447
Tags

Linked data

,

Foreign key

,

Business objects

Topics

Language Integrated Query - LINQ

,

MS SQL Server

,

.NET

Participating Experts
2
Points
500
Comments
7

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Answers

 

by: naspinskiPosted on 2009-10-07 at 07:42:55ID: 25516036

Definitely look into linq-t-sql (Linq-to-entities works too, but is a bit more complicated), it does lazy loading of all related data, it is very simple.  This is C#, but it will give you the idea.

//Get a specific company
Company company = db.companies.First(x => x.CompanyID == intGetThisID);
 
//now you have the company, want to get all of the orders
IEnumerable<Order> orders = company.Orders;
 
//its just that easy, want all the orders over $200?
IEnumerable<Order> ordersOver200 = company.Orders.Where(x => x.Amount > 200);
 
//or you could do this all at once
IEnumerable<Order> Comapny1OrdersOver200 = db.companies.First(c => c.CompanyID == 1).Orders.Where(o => o.Amount > 200);

                                              
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by: teebonPosted on 2009-10-07 at 07:50:03ID: 25516129

 

by: TinoNLPosted on 2009-10-07 at 08:29:03ID: 25516581

2 Naspinski: I have this working, after setting up a Linq to SQL class for the database. This is surely a very quick way to query your database, but the problem would be the lazy loading. Am I correct to think that each time that you access related data for an item, LINQ makes a roundtrip to the database?

In the real life situation, I have a list of > 10.000 companies, that have an average of 150 orders. All this data should be loaded into memory for further processing. Would this mean 10.001 database roundtrips?

 

by: naspinskiPosted on 2009-10-07 at 09:23:54ID: 25517142

No, it will write a SQL query automatically for you, so if you do this:

IEnumerable<Order> ordersOver200 = company.Orders.Where(x => x.Amount > 200);

It will do one trip, and load all those orders into memory.

What lazy loading means is that if you just load a company, it will not load the Orders unless you specifically ask for them.

 

by: naspinskiPosted on 2009-11-01 at 21:12:14ID: 25717167

I feel it has been answered quite sufficiently

20120131-EE-VQP-002

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