LinQ Queries

Friday, October 23, 2009

LINQ Queries
A query is an expression that retrieves data from a data source.
Queries are usually expressed in a specialized query language. Different languages have been developed over time for the various types of data sources, for example SQL for relational databases and XQuery for XML. Therefore, developers have had to learn a new query language for each type of data source or data format that they must support.

LINQ simplifies this situation by offering a consistent model for working with data across various kinds of data sources and formats. In a LINQ query, you are always working with objects. You use the same basic coding patterns to query and transform data in XML documents, SQL databases, ADO.NET Datasets, .NET collections, and any other format for which a LINQ provider is available.
Three Parts of a Query Operation

All LINQ query operations consist of three distinct actions:
1. Obtain the data source.
2. Create the query.
3. Execute the query.

The following example shows how the three parts of a query operation are expressed in source code. The example uses an integer array as a data source for convenience; however, the same concepts apply to other data sources also.

class IntroToLINQ
{
static void Main()
{
// The Three Parts of a LINQ Query:
// 1. Data source.
int[] numbers = new int[7] { 0, 1, 2, 3, 4, 5, 6 };
// 2. Query creation. // numQuery is an IEnumerable
var numQuery = from num in numbers
where (num % 2) == 0
select num;
// 3. Query execution.
foreach (int num in numQuery)
{ Console.Write("{0,1} ", num);
}
}
}