A LINQ query, often referred to as a query expression, consists of a combination of query clauses that identify the data sources and iteration variables for the query. A query expression can also include instructions for sorting, filtering, grouping, and joining, or calculations to apply to the source data. Query expression syntax resembles the syntax of SQL; therefore, you may find much of the syntax familiar.
A query expression starts with a From clause. This clause identifies the source data for a query and the variables that are used to refer to each element of the source data individually. These variables are named range variables or iteration variables. The From clause is required for a query, except for Aggregate queries, where the From clause is optional. After the scope and source of the query are identified in the From or Aggregate clauses, you can include any combination of query clauses to refine the query. For details about query clauses, see Visual Basic LINQ Query Operators later in this topic. For example, the following query identifies a source collection of customer data as the customers variable, and an iteration variable named cust.
Dim queryResults = From cust In customers _
Select cust.CompanyName
This example is a valid query by itself; however, the query becomes far more powerful when you add more query clauses to refine the result. For example, you can add a Where clause to filter the result by one or more values. Query expressions are a single line of code; you can just append additional query clauses to the end of the query. You can break up a query across multiple lines of text to improve readability by using the underscore (_) line-continuation character. The following code example shows an example of a query that includes a Where clause.
Dim queryResults = From cust In customers _
Where cust.Country = "USA"
Another powerful query clause is the Select clause, which enables you to return only selected fields from the data source. LINQ queries return enumerable collections of strongly typed objects. A query can return a collection of anonymous types or named types. You can use the Select clause to return only a single field from the data source. When you do this, the type of the collection returned is the type of that single field. You can also use the Select clause to return multiple fields from the data source. When you do this, the type of the collection returned is a new anonymous type. You can also match the fields returned by the query to the fields of a specified named type. The following code example shows a query expression that returns a collection of anonymous types that have members populated with data from the selected fields from the data source.
Dim queryResults = From cust In customers _
Where cust.Country = "USA" _
Select cust.CompanyName, cust.Country
LINQ queries can also be used to combine multiple sources of data and return a single result. This can be done with one or more From clauses, or by using the Join or Group Join query clauses. The following code example shows a query expression that combines customer and order data and returns a collection of anonymous types containing customer and order data.
Dim queryResults = From cust In customers, ord In orders _
Where cust.CustomerID = ord.CustomerID _
Select cust, ord
You can use the Group Join clause to create a hierarchical query result that contains a collection of customer objects. Each customer object has a property that contains a collection of all orders for that customer. The following code example shows a query expression that combines customer and order data as a hierarchical result and returns a collection of anonymous types. The query returns a type that includes a CustomerOrders property that contains a collection of order data for the customer. It also includes an OrderTotal property that contains the sum of the totals for all the orders for that customer. (This query is equivalent to a LEFT OUTER JOIN.)
Dim queryResults = From cust In customers _
Group Join ord In orders On _
cust.CustomerID Equals ord.CustomerID _
Into CustomerOrders = Group, _
OrderTotal = Sum(ord.Total) _
Select cust.CompanyName, cust.CustomerID, _
CustomerOrders, OrderTotal
There are several additional LINQ query operators that you can use to create powerful query expressions. The next section of this topic discusses the various query clauses that you can include in a query expression. For details about Visual Basic query clauses, see Queries (Visual Basic).