LINQ HAND BOOK FOR BEGINNERS

What is Linq

Linq is short for Language Integrated Query. If you are used to using SQL to query databases, you are going to have something of a head start with Linq, since they have many ideas in common. Before we dig into Linq itself, let’s step back and look at what makes SQL different from C#.

Syntax

var lQuery = from < object > in < table_Name or List_Name > where < object > . < column_name > < relational operator > < Condition_to_satisfied > select < object > . < column_name > ;

eg

Var lQuery=from o in Orders
where o.CustomerID == 84
select o.Cost;

Computed column

Note that you can perform whatever computation you wish inside the anonymous type initializer.

eg

var lQuery= from o in Orders
where o.CustomerID == 84 select new { o.OrderID, o.Cost, CostWithTax = o.Cost * 10 };

Computed column with check constraint

eg

var lQuery= from o in Orders
where o.CustomerID == 84 && o.Cost > 100
select new { o.OrderID, o.Cost, CostWithTax = o.Cost * 10 };

Ordering (sorting)

This is achieved by using the new “orderby” keyword.

eg

var lQuery= from o in Orders
where o.CustomerID == 84
orderby o.Cost ascending
select new { o.OrderID, o.Cost };
var Found = from o in Orders
where o.CustomerID == 84
orderby o.Cost descending select new { o.OrderID, o.Cost };

Getting Values From multiple Tables

It is achieved by using the “from” keyword multiple times.

eg

var lQuery = from o in Orders
from c in Customers
where o.CustomerID == c.CustomerID
select new { c.Name, o.OrderID, o.Cost };

Grouping

A list of matching objects,

eg

// Group orders by customer.
var lQuery = from o in Orders
group o.Cost by o.CustomerID;
// Iterate over the groups.
foreach(var Cust in lQuery)
{
    // About the customer…
    Console.WriteLine(“Customer with ID” + Cust.Key.ToString() + ”ordered” + Cust.Count().ToString() + ”items.”);
    // And the costs of what they ordered.
    foreach(var Cost in Cust)
    Console.WriteLine(”Cost: ” + Cost.ToString());
}