Tài liệu Displaying Columns from a Related DataTable doc

The sample code creates a new DataSet containing the Orders table and the Order Details table from Northwind.. A column is added to the Order Details table that gets the CustomerID from

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Recipe 2.17 Displaying Columns from a Related DataTable

Problem

You want to add a column to a DataTable that displays a value from a row in a related table in the DataSet

Solution

Use expression columns to retrieve lookup values based on DataRelation objects

The sample code creates a new DataSet containing the Orders table and the Order Details table from Northwind A DataRelation is created between the tables A column is added

to the Order Details table that gets the CustomerID from the parent Order using the

relation created between the tables Finally, the default view of the Orders table is bound

to the data grid on the form

The C# code is shown in Example 2-22

Example 2-22 File: LookupColumnsForm.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";

private const String CUSTOMERID_FIELD = "CustomerID";

private const String QUANTITY_FIELD = "Quantity";

//

DataSet ds = new DataSet( );

// Fill the Orders table and add it to the DataSet

SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Orders",

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

DataTable ordersTable = new DataTable(ORDERS_TABLE);

da.Fill(ordersTable);

ds.Tables.Add(ordersTable);

// Fill the OrderDetails table and add it to the DataSet

da = new SqlDataAdapter("SELECT * FROM [Order Details]",

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

DataTable orderDetailsTable = new DataTable(ORDERDETAILS_TABLE);

da.Fill(orderDetailsTable);

ds.Tables.Add(orderDetailsTable);

// Create a relation between the tables

ds.Relations.Add(ORDERS_ORDERDETAILS_RELATION,

ordersTable.Columns[ORDERID_FIELD],

orderDetailsTable.Columns[ORDERID_FIELD],

true);

// Add a column to Orders for the total items in all Order Detail rows

ordersTable.Columns.Add("TotalQuantity", typeof(int),

"Sum(Child." + QUANTITY_FIELD + ")");

// Add a column to Order Details getting CustomerID from Order parent

orderDetailsTable.Columns.Add(CUSTOMERID_FIELD, typeof(string),

"Parent(" + ORDERS_ORDERDETAILS_RELATION + ")." +

CUSTOMERID_FIELD);

// Bind the default view of the Order table to the grid

resultDataGrid.DataSource = ordersTable.DefaultView;

Discussion

An expression column creates a calculated column that displays information from a related record You can refer to a column in a parent record by affixing Parent to the beginning of the name of the column in the parent table If there are multiple parent records because of multiple data relations, the name of the DataRelation is specified

within parentheses and Parent(DataRelationName) is affixed to the name of the column

in the parent table

The sample code creates a calculated column in the Order Details table retrieving the

CustomerID from the parent Orders record for each Order Details record:

orderDetailsTable.Columns.Add(CUSTOMERID_FIELD, typeof(string),

"Parent(" + ORDERS_ORDERDETAILS_RELATION + ")." +

CUSTOMERID_FIELD);

When the constants are replaced with their values, this code is equivalent to:

orderDetailsTable.Columns.Add("CustomerID", typeof(string),

"Parent(Orders_OrderDetails_Relation).CustomerID");

Similarly, you can refer to child records within an expression by affixing the beginning of

the column name in the child table with Child or Child(DataRelationName) depending

on whether the row has child records from multiple tables Additionally, since multiple

child records can exist for a parent record, the reference to the child column must be used within an aggregate function Supported aggregates are listed in Table 2-15

Table 2-15 Aggregate functions supported in DataColumn expressions

Aggregate Description

Avg Average of all values

StDev Statistical standard deviation of all values

Var Statistical variance of all values

The sample code creates a calculated column in the Orders table for the total quantity of all order detail items for an order with the following code:

ordersTable.Columns.Add("TotalQuantity", typeof(int),

"Sum(Child." + QUANTITY_FIELD + ")");

When the constants are replaced with their values, the code is:

ordersTable.Columns.Add("TotalQuantity", typeof(int),

"Sum(Child.Quantity)");

Unlike the sample code referencing a parent row, the DataRelation name is omitted

because only one DataRelation exists and it is unnecessary in this case

If the parent record has no child records, the aggregate function returns a null reference in C# or Nothing in VB.NET

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