# Aggregate Clause (Visual Basic)

**Visual Studio 2015**

Applies one or more aggregate functions to a collection.

Aggregate element [As type] In collection _ [, element2 [As type2] In collection2, [...]] [ clause ] Into expressionList

Term | Definition |

| Required. Variable used to iterate through the elements of the collection. |

| Optional. The type of |

| Required. Refers to the collection to operate on. |

| Optional. One or more query clauses, such as a |

| Required. One or more comma-delimited expressions that identify an aggregate function to apply to the collection. You can apply an alias to an aggregate function to specify a member name for the query result. If no alias is supplied, the name of the aggregate function is used. For examples, see the section about aggregate functions later in this topic. |

The **Aggregate** clause can be used to include aggregate functions in your queries. Aggregate functions perform checks and computations over a set of values and return a single value. You can access the computed value by using a member of the query result type. The standard aggregate functions that you can use are the **All**, **Any**, **Average**, **Count**, **LongCount**, **Max**, **Min**, and **Sum** functions. These functions are familiar to developers who are familiar with aggregates in SQL. They are described in the following section of this topic.

The result of an aggregate function is included in the query result as a field of the query result type. You can supply an alias for the aggregate function result to specify the name of the member of the query result type that will hold the aggregate value. If no alias is supplied, the name of the aggregate function is used.

The **Aggregate** clause can begin a query, or it can be included as an additional clause in a query. If the **Aggregate** clause begins a query, the result is a single value that is the result of the aggregate function specified in the **Into** clause. If more than one aggregate function is specified in the **Into** clause, the query returns a single type with a separate property to reference the result of each aggregate function in the **Into** clause. If the **Aggregate** clause is included as an additional clause in a query, the type returned in the query collection will have a separate property to reference the result of each aggregate function in the **Into** clause.

The following list describes the standard aggregate functions that can be used with the **Aggregate** clause.

Function | Description |

| Returns Dim customerList1 = Aggregate order In orders Into AllOrdersOver100 = All(order.Total >= 100) |

| Returns |

| Computes the average of all elements in the collection, or a computes supplied expression for all elements in the collection. Following is an example: Dim customerOrderAverage = Aggregate order In orders Into Average(order.Total) |

| Counts the number of elements in the collection. You can supply an optional |

| Refers to query results that are grouped as a result of a |

| Counts the number of elements in the collection. You can supply an optional |

| Computes the maximum value from the collection, or computes a supplied expression for all elements in the collection. Following is an example: Dim customerMaxOrder = Aggregate order In orders Into MaxOrder = Max(order.Total) |

| Computes the minimum value from the collection, or computes a supplied expression for all elements in the collection. Following is an example: |

| Computes the sum of all elements in the collection, or computes a supplied expression for all elements in the collection. Following is an example: Dim customerTotals = From cust In customers Aggregate order In cust.Orders Into Sum(order.Total) |

The following code example shows how to use the **Aggregate** clause to apply aggregate functions to a query result.

Public Sub AggregateSample() Dim customers = GetCustomerList() Dim customerOrderTotal = From cust In customers Aggregate order In cust.Orders Into Sum(order.Total), MaxOrder = Max(order.Total), MinOrder = Min(order.Total), Avg = Average(order.Total) For Each customer In customerOrderTotal Console.WriteLine(customer.cust.CompanyName & vbCrLf & vbTab & "Sum = " & customer.Sum & vbCrLf & vbTab & "Min = " & customer.MinOrder & vbCrLf & vbTab & "Max = " & customer.MaxOrder & vbCrLf & vbTab & "Avg = " & customer.Avg.ToString("#.##")) Next End Sub

You can include your own custom aggregate functions in a query expression by adding extension methods to the IEnumerable<T> type. Your custom method can then perform a calculation or operation on the enumerable collection that has referenced your aggregate function. For more information about extension methods, see Extension Methods (Visual Basic).

For example, the following code example shows a custom aggregate function that calculates the median value of a collection of numbers. There are two overloads of the Median extension method. The first overload accepts, as input, a collection of type **IEnumerable(Of Double)**. If the Median aggregate function is called for a query field of type **Double**, this method will be called. The second overload of the Median method can be passed any generic type. The generic overload of the Median method takes a second parameter that references the **Func(Of T, Double)** lambda expression to project a value for a type (from a collection) as the corresponding value of type **Double**. It then delegates the calculation of the median value to the other overload of the Median method. For more information about lambda expressions, see Lambda Expressions (Visual Basic).

Imports System.Runtime.CompilerServices Module UserDefinedAggregates ' Calculate the median value for a collection of type Double. <Extension()> Function Median(ByVal values As IEnumerable(Of Double)) As Double If values.Count = 0 Then Throw New InvalidOperationException("Cannot compute median for an empty set.") End If Dim sortedList = From number In values Order By number Dim medianValue As Double Dim itemIndex = CInt(Int(sortedList.Count / 2)) If sortedList.Count Mod 2 = 0 Then ' Even number of items in list. medianValue = ((sortedList(itemIndex) + sortedList(itemIndex - 1)) / 2) Else ' Odd number of items in list. medianValue = sortedList(itemIndex) End If Return medianValue End Function ' "Cast" the collection of generic items as type Double and call the ' Median() method to calculate the median value. <Extension()> Function Median(Of T)(ByVal values As IEnumerable(Of T), ByVal selector As Func(Of T, Double)) As Double Return (From element In values Select selector(element)).Median() End Function End Module

The following code example shows sample queries that call the Median aggregate function on a collection of type **Integer**, and a collection of type **Double**. The query that calls the Median aggregate function on the collection of type **Double** calls the overload of the Median method that accepts, as input, a collection of type **Double**. The query that calls the Median aggregate function on the collection of type **Integer** calls the generic overload of the Median method.

Module Module1 Sub Main() Dim numbers1 = {1, 2, 3, 4, 5} Dim query1 = Aggregate num In numbers1 Into Median(num) Console.WriteLine("Median = " & query1) Dim numbers2 = {1.9, 2, 8, 4, 5.7, 6, 7.2, 0} Dim query2 = Aggregate num In numbers2 Into Median() Console.WriteLine("Median = " & query2) End Sub End Module