SelectNode Class
A node that provides data with a SELECT query.
Namespace: DevExpress.DataAccess.DataFederation
Assembly: DevExpress.DataAccess.v24.2.dll
NuGet Package: DevExpress.DataAccess
Declaration
Related API Members
The following members return SelectNode objects:
Remarks
The SelectNode objects are contained in the QueryNodeCollection accessible with the FederationDataSourceBase.Queries property.
You can create a Join federated query in the following ways:
Fluent Interface
This method uses the SelectNodeBuilder object that provides a fluent interface to build a query tree. A fluent interface allows you to create queries in a few lines of code.
The entry point is the Source.From method. It returns the SelectNodeBuilder instance.
The final point is the SelectNodeBuilder.Build method that returns the query object.
Follow the steps below to build a federated query:
- Create a Source object for each data source integrated in the federated data source.
- Choose the source that becomes the query root and call its Source.From extension method.
- Chain SelectNodeBuilder method calls, such as Select and Join, to build a query.
- Call the SelectNodeBuilder.Build method to finalize the process.
- Assign the resulting query to the
SelectNodeinstance. - Add the SelectNode object to the FederationDataSourceBase.Queries collection.
The following code sample creates a federated data source with two federated queries from SQL and Excel data sources.
using DevExpress.DataAccess.DataFederation;
// ...
FederationDataSource federation = new FederationDataSource();
Source sourceProducts = new Source("Products", CreateSqlDataSource(), "Products");
Source sourceOrderDetail = new Source("OrderDetail", CreateExcelDataSource("SalesPerson.xlsx", "Data"));
Source sourceHeader = new Source("OrderHeader", CreateExcelDataSource("OrderHeaders.xlsx", "OrderHeader"));
SelectNode query1 = sourceProducts.From().Select("ProductName", "QuantityPerUnit", "UnitsInStock")
.Join(sourceOrderDetail, "[Products.ProductName] = [OrderDetail.ProductName]")
.Select("OrderDate")
.Build("ProductsOrderDetail");
SelectNode query2 = sourceHeader.From().Select("OrderID", "Status", "Description")
.Join(sourceOrderDetail, "[OrderHeader.OrderID] = [OrderDetail.OrderID]")
.Select("Quantity", "Extended Price")
.Build("OrderHeaderOrderDetail");
federation.Queries.AddRange(new[] { query1, query2 });
federation.Fill();
Data Federation API
This method builds a query tree by instantiating objects and adding them to the collections.
Follow the steps below to build a federated query:
- Create a Source object for each data source integrated in the federated data source.
- Create a a SourceNode object for each data source.
- Choose the source node that becomes the query root and create a
SelectNodeobject for that data source. - Create SelectColumnExpression objects for each data field and add it to the SelectNode.Expressions collection.
- Create JoinElement objects for each JOIN and add it to the SelectNode.SubNodes collection.
- Add the SelectNode object to the FederationDataSourceBase.Queries collection.
The following code sample creates a federated data source with a federated query from SQL and Excel data sources.
using DevExpress.DataAccess.DataFederation;
// ...
FederationDataSource federation = new FederationDataSource();
Source sourceProducts = new Source("Products", CreateSqlDataSource(), "Products");
Source sourceOrderDetail = new Source("OrderDetail", CreateExcelDataSource("SalesPerson.xlsx", "Data"));
var sourceNodeProducts = new SourceNode(sourceProducts, "Products");
var sourceNodeOrderDetail = new SourceNode(sourceOrderDetail, "OrderDetail");
var query = new SelectNode(sourceNodeProducts)
{
Alias = "ProductsOrderDetail",
SubNodes = {
new JoinElement {
Node = sourceNodeOrderDetail,
Condition = $"[{sourceNodeProducts.Alias}.ProductName] == [{sourceNodeOrderDetail.Alias}.ProductName]"
}
},
Expressions = {
new SelectColumnExpression(sourceNodeProducts, "ProductName"),
new SelectColumnExpression(sourceNodeProducts, "QuantityPerUnit"),
new SelectColumnExpression(sourceNodeOrderDetail, "OrderID"),
new SelectColumnExpression(sourceNodeOrderDetail, "OrderDate"),
new SelectColumnExpression(sourceNodeOrderDetail, "Quantity"),
new SelectColumnExpression(sourceNodeOrderDetail, "UnitPrice")
}
};
federation.Queries.Add(query);
federation.Fill();
Example
This code snippet demonstrates how to create a federated data source that uses a filter with a parameter.
Note
The complete sample project How to Create a Federated Data Source at Runtime is available in the DevExpress Examples repository.
using DevExpress.DataAccess.DataFederation;
// ...
FederationDataSource federation = new FederationDataSource();
Source source = new Source("excelSource", CreateExcelDataSource("SalesPerson.xlsx", "Data"));
var sourceNode = new SourceNode(source, "Orders");
var query = new SelectNode(sourceNode)
{
Alias = "excel",
Expressions = {
new SelectColumnExpression(sourceNode, "OrderID"),
new SelectColumnExpression(sourceNode, "OrderDate"),
new SelectColumnExpression(sourceNode, "Sales Person"),
new SelectColumnExpression(sourceNode, "ProductName"),
new SelectColumnExpression(sourceNode, "Extended Price")
},
FilterString = "[Orders.CategoryName] = ?cat",
};
federation.Queries.Add(query);
federation.Fill(new[] {
new DevExpress.DataAccess.Sql.QueryParameter("cat", typeof(string), "Seafood") });