*/ DataTable myTable = m_ds...
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Smutek to uczucie, jak gdyby się tonęło, jak gdyby grzebano cię w ziemi.
Tables["OrderDetail"];
/* Display all column names. */
foreach(DataColumn c in myTable.Columns) {
Console.Write(c.ColumnName + "\t");
}
Console.WriteLine(""); // newline
/* Process each row. */
foreach(D ataRow r in myTable.Rows) {
/* Display each column. */
foreach(DataColumn c in myTable.Columns) {
Console.Write(r[c] + "\t");
}
Console.WriteLine(""); // newline
}
Here is the output of that code:
fk_OrderID ProductCode Quantity Pric e
101 Item-100 12 59.95
101 Item-200 1 9.25
102 Item-200 3 9.25
Typically, a DataTable has one or more fields serving as a primary key. This functionality is exposed as the Primar yKey property. Because the primary key might contain more than one field, this property is an array of DataColumn objects. We revisit this excerpt of code here to put things in context. Note that in this example, the primary key consists of only one field, hence the array of size one.
// Register the column "OrderID" as the primary key of table "Order"
DataColumn[] keys = new DataColumn[1];
keys[0] = m_ds.Tables["Order"].Columns["OrderID"];
m_ds.Tables["Order"].PrimaryKey = keys;
5.3.2.1 Relations and constraints
Relations define how tables in a database relate to each other. The DataSet globally stores the collection of relations between tables in the Relations property; however, each of the tables participating in the relation also has to know about the r elationship. ChildRelations and ParentRelations, two properties of the DataTable object, take care of this. ChildRelations enumerates all relations that this table participates in as a master table. ParentRelations, on the other hand, lists the relations i n which this table acts as a slave table. We provide more information on the topic of relations when we explain the DataRelation object in an upcoming section of this chapter.
While we are on the topic of tables and relationships, it is important to understand how to set up constraint enforcements. There are two types of constraints that we can set up and enforce, UniqueConstraint and ForeignKeyConstraint. UniqueConstraint enforces the uniqueness of a field value for a table. ForeignKeyConstraint enforces rules on table relationships. For ForeignKeyConstraint, we can set up UpdateRule and DeleteRule to dictate how the application should behave upon performing update or delete on a row of data in the parent table.
Table 5-1 shows the constraint settings and behavior of ForeignKeyConstraint rules.
95
Table 5-1. Constraint types and behaviors
Setting
Behavior
None
Nothing.
Dependent rows (identified by foreign key) are deleted/updated when parent row is Cascade deleted/updated.
SetDefault Foreign keys in dependent rows are set to the default value when parent row is deleted.
SetNull
Foreign keys in dependent rows are set to null value when parent row is deleted.
Constraints are activated only when the EnforceConstraint property of the DataSet object is set to true.
5.3.3 DataView
The DataView object is similar to a view in conventional database programming. We can create different customized views of a DataTable, each having different sorting and filtering criteria. Through these different views, we can traverse, search, and edit individual records. This ADO.NET concept is the closest to the old ADO recordset. In ADO.NET, DataView serves another important role—d ata binding to Windows Forms and Web Forms. We show the usage of DataView when we discuss data binding on Windows Forms and Web Forms in Chapter 7 and Chapter 8.
5.3.4 DataRelation
A DataSet object as a collection of DataTable objects alone is not useful enough. A collection of DataTable objects returned by a server component provides little im provement upon the chained recordset in previous versions of ADO. In order for your client application to make the most of the returned tables, you also need to return the relations between these DataTables. This is where the DataRelation object comes into play.
With DataRelation, you can define relationships between the DataTable objects. Client components can inspect an individual table or navigate the hierarchy of tables through these relationships. For example, you can find a particular row in a parent table and then traverse all dependent rows in a child table.
The DataRelation contains the parent table name, the child table name, the parent table column (primary key), and the child table column (foreign key).
By having multiple DataTables and DataRelations within the DataSet, ADO.NET allows for a much more flexible environment where consumers of the data can choose to use the data in whichever way they wish.
One example might be the need to display all information about a particular parent table and all of its dependent rows in a child table. You have ten rows in the parent table. Each of the rows in the parent table has ten dependent rows in the child table. Let's consider two approaches to getting this data to the data consumer. First, we will just use a join in the query string:
Select
Order.CustomerFirstName, Order.CustomerLastName, Order.OrderDate,
OrderDetail.ProductCode, OrderDetail.Quantity, OrderDetail.Price
from
Order, OrderDetail
where Order.OrderID = OrderDetail.fk_OrderID
The result set contains 100 rows, in which each group of ten rows contains duplicate information about the parent row.
96
.NET Framework Essentials
A second approach is to retrieve the list of rows from the parent table first, which would be ten rows: Select
Order.OrderID,
Order.CustomerFirstName, Order.CustomerLastName, Order.OrderDate
from
Order
Then for each of the ten rows in the parent table, you would retrieve the dependent rows from the child table:
Select
OrderDetail.ProductCode, OrderDetail.Quantity, OrderDetail.Price
from
OrderDetail where fk_OrderID = thisOrderID
This second approach is less of a resource hog since there is no redundant data; however, you end up making 11 round-trips (one time for the parent table, and 10 times for each parent of the child table).
It's better to get the parent table, the child table, and the relation between them using one round-trip, without all the redundant data. This is one of the biggest benefits that DataSet brings. The following block of code demonstrates the power of having ta bles and relationships:
/*
* Given an order id, display a single order.
*/
public static void DisplaySingleOrder(DataSet m_ds, int iOrderID) {
Decimal runningTotal = 0;
Decimal lineTotal = 0;
Decimal dPrice = 0;