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Querying with Typed DataSet Objects
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Recall that a typed DataSet inherits from the DataSet class, so it functionally is a DataSet that contains multiple DataTable objects. However, these DataTable objects are exposed as strongly typed objects, collections, and fields. This makes the LINQ queries you write strongly typed too. You do not need to use the Field method. Instead, you can query directly against the Tablename.Fieldname construct. As an example, suppose you created the typed DataSet shown back in Figure 11-3. You can load this typed DataSet by passing it to a DataAdapter instance. You can then use LINQ to DataSet to write a strongly typed LINQ query against the cached results. The following code shows just that. The results are then bound to a GridView control.
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Sample of Visual Basic Code Dim pubsCnn As ConnectionStringSettings = _ ConfigurationManager.ConnectionStrings("PubsData") Dim pubs As PubsDataSet = New PubsDataSet() Dim adp As SqlDataAdapter = New SqlDataAdapter( "select * from publishers;", pubsCnn.ConnectionString) adp.Fill(pubs, "publishers") Dim pubsQuery As IEnumerable(Of DataRow) = From p In pubs.publishers Where p.country = "USA" Select p GridView1.DataSource = pubsQuery GridView1.DataBind() Sample of C# Code ConnectionStringSettings pubsCnn = ConfigurationManager.ConnectionStrings["PubsData"];
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660 ChAPTER 11
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Connecting to and Querying Data with LINQ
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PubsDataSet pubs = new PubsDataSet(); SqlDataAdapter adp = new SqlDataAdapter( "select * from publishers;", pubsCnn.ConnectionString); adp.Fill(pubs, "publishers"); IEnumerable<DataRow> pubsQuery = from p in pubs.publishers where p.country == "USA" select p; GridView1.DataSource = pubsQuery; GridView1.DataBind();
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Cross-Table LINQ to DataSet Queries
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You can use the LINQ join feature discussed in Lesson 1 to join two different DataSet objects in a single query and produce a new result set. The following code shows an example. First, the DataSet is loaded with two tables from the pubs database (publishers and titles). Note that this code is not shown but simply uses a DataAdapter to fill two different tables in the same DataSet. Next, both DataTable objects are set to variables. These variables are used in the LINQ query in the join clause. Notice that the tables are simply joined on the pub_id field. The results of the query are then pushed into a new anonymous type. Finally, the query is bound to a GridView control for display to a user.
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Sample of Visual Basic Code Dim dbSchema As DatabaseSchema = New DatabaseSchema() Dim pubs As DataSet = dbSchema.GetPubsData() Dim publishers As DataTable = pubs.Tables("publishers") Dim titles As DataTable = pubs.Tables("titles") Dim pubQuery = From title In titles.AsEnumerable() Join pub In publishers.AsEnumerable() On title.Field(Of String)("pub_id") Equals pub.Field(Of String)("pub_id") Where title.Field(Of Decimal)("price") < 10 Order By title.Field(Of String)("title") Select New With { .Publisher = pub.Field(Of String)("pub_name"), .Title = title.Field(Of String)("title"), .Price = title.Field(Of Decimal)("price") } GridView1.DataSource = pubQuery GridView1.DataBind()
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Lesson 2: LINQ and ADO.NET
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ChAPTER 11
Sample of C# Code DatabaseSchema dbSchema = new DatabaseSchema(); DataSet pubs = dbSchema.GetPubsData(); DataTable publishers = pubs.Tables["publishers"]; DataTable titles = pubs.Tables["titles"]; var pubQuery = from title in titles.AsEnumerable() join pub in publishers.AsEnumerable() on title.Field<string>("pub_id") equals pub.Field<string>("pub_id") where title.Field<decimal>("price") < 10 orderby title.Field<string>("title") select new { Publisher = pub.Field<string>("pub_name"), Title = title.Field<string>("title"), Price = title.Field<decimal>("price") }; GridView1.DataSource = pubQuery; GridView1.DataBind();
Comparing Data in DataSet Objects
You can use LINQ features to compare data contained in one or more DataTables. These features include the following operators:
Distinct Union Except intersect
Used to return distinct DataRows in a collection Joins two like DataTable objects together Returns a collection of DataRow objects that appear in both DataTable objects Returns those DataRow objects that are different between two DataTable objects
You use the DataRowComparer when using these operators. This ensures that DataRows are compared against one another for equal values across columns. In the following example, a DataSet is created from the titles table in the pubs database. A LINQ query then runs to get all price values for books that have sold more than 1000 copies. The Distinct method is then used to show a list of distinct prices across this list of books.
Sample of Visual Basic Code Dim dbSchema As DatabaseSchema = New DatabaseSchema() Dim titlesDs As DataSet = dbSchema.GetTitles() Dim titleQuery = From title In titlesDs.Tables("titles") Where title.Field(Of Integer)("ytd_sales") > 1000 Order By title.Field(Of Decimal)("price") Select title.Field(Of Decimal)("price") Dim prices = titleQuery.Distinct() GridView1.DataSource = prices GridView1.DataBind()
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