Recipe 3.7 Using Expression Columns to Display Aggregate Values

Problem

You want to add summary information such as averages, sums, and counts to a table based on related child rows.

Solution

Use expression columns to perform aggregate calculations based on child rows.

The sample code starts by creating a DataSet containing the Orders and Order Details tables from Northwind sample database and a relation between them. An expression is added to the Order Details table to calculate the extended price for each row. Aggregate values for the total extended price of the order and the number of Order Detail rows are added to the Orders table. Finally, the default view of the Orders table is bound to the data grid to display the results.

The C# code is shown in Example 3-7.

Example 3-7. File: ChildAggregateForm.cs

// Namespaces, variables, and constants
using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

// Table name constants
private const String ORDERS_TABLE       = "Orders";
private const String ORDERDETAILS_TABLE = "OrderDetails";

// Relation name constants
private const String ORDERS_ORDERDETAILS_RELATION =
    "Orders_OrderDetails_Relation";

// Field name constants
private const String ORDERID_FIELD      = "OrderID";

//  . . . 

DataSet ds = new DataSet( );

SqlDataAdapter da;

// Fill the Order table and add it to the DataSet.
da = new SqlDataAdapter("SELECT * FROM Orders",
    ConfigurationSettings.AppSettings["Sql_ConnectString"]);
DataTable orderTable = new DataTable(ORDERS_TABLE);
da.Fill(orderTable);
ds.Tables.Add(orderTable);

// Fill the OrderDetails table and add it to the DataSet.
da = new SqlDataAdapter("SELECT * FROM [Order Details]",
    ConfigurationSettings.AppSettings["Sql_ConnectString"]);
DataTable orderDetailTable = new DataTable(ORDERDETAILS_TABLE);
da.Fill(orderDetailTable);
ds.Tables.Add(orderDetailTable);

// Create a relation between the tables.
ds.Relations.Add(ORDERS_ORDERDETAILS_RELATION,
    ds.Tables[ORDERS_TABLE].Columns[ORDERID_FIELD],
    ds.Tables[ORDERDETAILS_TABLE].Columns[ORDERID_FIELD],
    true);

// Create the expression column for the line total.
orderDetailTable.Columns.Add("OrderDetailTotal", typeof(Decimal),
    "(Quantity * UnitPrice) * (1-Discount)");
// Create the OrderDetails aggregate values in the Order table.
orderTable.Columns.Add("OrderDetailCount", typeof(int),
    "COUNT(Child.ProductId)");
orderTable.Columns.Add("OrderTotal", typeof(Decimal),
    "SUM(Child.OrderDetailTotal)");

// Bind the DataSet to the grid.
childAggregateDataGrid.DataSource = ds.DefaultViewManager;
childAggregateDataGrid.DataMember = ORDERS_TABLE;

Discussion

You can create aggregate columns within a table to display summary information for related child records. When a DataRelation exists between a parent and child table in a DataSet, you can refer to a child record by adding the prefix Child. to the column name in the child table. In the sample code, COUNT(Child.ProductID) returns the number of Order Details child records for the parent Order record. Expression columns support aggregate functions as shown in Table 3-2.

Table 3-2. Aggregate functions supported by expression columns

Function	Description
AVG	Average of all values
COUNT	Number of values
MAX	Largest value
MIN	Smallest value
STDEV	Statistical standard deviation of all values
SUM	Sum of all values
VAR	Statistical variance of all values

If the parent table has more than one child table, the relationship must be specified in the aggregate function. The fully qualified syntax to access the count of child Order Details records would be:

COUNT(Child("Orders_OrderDetails_Relation").ProductId)

You can refer to the parent table for a child in a similar manner by adding the prefix Parent. to the column name. In the previous example, for a row in the Order Details table, Parent.CustomerID refers to the CustomerID for the parent Orders row.