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XML docum ent s and ADO.NET dat a set s int eract wit h one anot her in m ult iple way s. Underst anding t hese int eract ions and k now ing how t o put t hem t o use can help you quer y and m anipulat e dat a bot h locally on a client s w or kst at ion and on a dat abase ser ver. This sect ion pr ov ides a select ion of sam ples t o show how t o use XML docum ent s wit h dat a set s for t hese purposes. As w it h m any t opics addressed by t his book, t he pr esent at ion isn t m eant t o prov ide ex haust ive coverage of ev ery possible feat ur e on a t opic. I nst ead, t he sect ion aim s t o prov ide a firm foundat ion t hat w ill equip you t o go on and learn m ore in what ev er dir ect ions y our needs dict at e.
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Cr e a t in g H ie r a r ch ica l X M L D ocum e n t s
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One of t he really valuable aspect s of t he Dat aSet obj ect in ADO.NET is t hat it is XML- based. What t his m eans is t hat you can m anipulat e t he elem ent s wit hin a dat a set and indir ect ly m odify XML st r uct ures. This feat ure is part icular ly beneficial w hen w ork ing wit h m ult it able row sources t hat hav e parent - child relat ionships because it reliev es dev elopers fr om r epresent ing t hese com plex relat ionships in XSD schem as. Alt hough ADO.NET and XML are relat iv ely new t o m any Visual Basic developers, t he obj ect m odel for dat a set s in ADO.NET m akes it relat iv ely m ore fam iliar t o t hose w it h any background in m anipulat ing obj ect s. See Chapt er 10 for a general rev iew of ADO.NET obj ect s. Figure 10- 1 prov ides an ov erv iew of t he Dat aSet obj ect m odel, and num erous code sam ples t hr oughout Chapt er 10 dem onst rat e ADO.NET pr ogram m ing t opics, including t he Dat aSet obj ect and it s hierarchically dependent obj ect s. A Dat aSet obj ect and it s associat ed XML docum ent are lik e t w o sides of t he sam e coin. Wit h t he Wr it eXm l m et hod for a Dat aSet obj ect , you can persist bot h t he cont ent s of an XML docum ent and t he under ly ing schem a for t he docum ent . I n addit ion, when a dat a set has changes not com m it t ed t o a r em ot e dat abase, y ou can generat e v ia t he Wr it eXm l m et hod t he DiffGram represent ing t he dat a set wit h it s uncom m it t ed changes. Recall t hat a DiffGram cont ains cur rent v alues as well as prev ious values. The DiffGram is readily available because ADO.NET conv eys changes from a client t o a SQL Ser ver inst ance v ia DiffGram s. The sam ple in t his sect ion dem onst rat es how t o creat e a t hr ee- t ier ed dat a set based on t hr ee t ables fr om t he Nort hw ind dat abase. These t ables are t he Cust om ers, Orders, and Order Det ails t ables. I ndiv idual cust om ers ar e parent s of indiv idual orders, and orders, in t ur n, are par ent s of order det ails, or line it em s wit hin an order. This pair of nest ed r elat ions is t he k ind of st r uct ur e t hat XML docum ent s r epr esent especially w ell because t he docum ent shows t he act ual nest ing inst ead of a single flat rowset . The sam ple r elies on t w o pr ocedur es. The first procedur e, Sav eThreeTierDasAsXm lDocum ent , calls a second procedure t hat generat es a dat a set and t hen persist s t he dat a set as an XML docum ent . By using t he Writ eXm l m et hod, t he SaveThreeTierDasAsXm lDocum ent procedure av oids a reliance on SQLXML Managed Classes. This m eans t he t echniques dem onst rat ed in t his chapt er ar e relat ively r obust in t hat t hey can work w it h any dat a source t o which ADO.NET can connect . I n addit ion, t he pr ocedur es dem onst rat ed for t he Dat aSet obj ect don t r equir e t he inst allat ion of eit her Web Release 2 or Web Release 3, as is necessary for t he use of Managed Classes. The second procedur e, Creat eThreeTierDat aSet , is a funct ion procedur e t hat ret ur ns a Dat aSet obj ect t o t he procedure t hat calls it . I t is t his r et ur ned dat a set t hat t he first pr ocedur e persist s as an XML docum ent in a file. The Sav eThreeTierDasAsXm lDocum ent pr ocedure st art s by inst ant iat ing a Dat aSet obj ect and populat ing it wit h t he dat a set ret urned by t he Cr eat eThreeTierDat aSet funct ion procedure. Aft er populat ing t he dat a set , t he procedur e prepar es t o persist it as a file w it h Unicode charact ers. These act ions t ake several st eps. The procedur e st art s t he process by assigning t he nam e of t he XML docum ent t o a st ring var iable ( st r1) . Next t he pr ocedur e inst ant iat es a FileSt ream obj ect ( fst 1) t o hold t he file cont aining t he XML docum ent . Then t he procedur e inst ant iat es an Xm lText Wr it er obj ect ( t xw1) t o copy t he XML wit hin t he dat a set t o t he FileSt r eam obj ect . The Wr it eXm l m et hod uses t xw1 as one of it s t wo argum ent s for copy ing t he XML fr om t he dat a set t o t he file. The ot her argum ent , which is Xm lWr it eMode.Writ eSchem a in t his case, det erm ines how t he Writ eXm l m et hod conv eys cont ent fr om t he dat a set t o t he file. The Xm lWr it eMode.Wr it eSchem a argum ent dir ect s t he Writ eXm l m et hod t o st art by copying t he schem a for t he docum ent and t hen follow t he schem a w it h t he cont ent s of t he XML docum ent . Aft er w r it ing t he docum ent , t he procedure fr ees resources and ret urns cont rol t o t he pr ocedur e by closing bot h t he Xm lText Wr it er and FileSt ream obj ect s.
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The Creat eThreeTierDat aSet pr ocedur e st art s by inst ant iat ing a connect ion obj ect and opening it so t hat t he connect ion point s t o t he Nort hw ind dat abase. The procedur e next inst ant iat es a Dat aSet obj ect ( das1) and uses t he connect ion obj ect t o connect a SqlDat aAdapt er obj ect ( dap1) wit h t he Cust om ers t able in t he Nort hwind dat abase. Then t he procedure copies t he Cust om ers t able rows int o a dat a t able nam ed Cust om ers w it hin das1 by inv ok ing t he Fill m et hod for t he dap1 obj ect . Aft er adding t he Cust om ers t able from t he Nort hw ind dat abase t o t he das1 dat a set , t he pr ocedur e point s dap1 t o t he Orders t able in t he Nort hw ind dat abase. Then it adds t he Orders t able t o das1. I t repeat s t he pr ocess a t hird and final t im e t o creat e an OrderDet ails dat a t able in das1 wit h t he colum n values from t he Order Det ails t able in t he Nort hw ind dat abase. At t he end of t hese t hr ee invocat ions of t he Fill m et hod, t he das1 dat a set cont ains t hr ee unrelat ed t ables. How ev er, w e need Dat aRelat ion obj ect s t o specify t he hierarchical relat ionship bet w een t ables. I n fact , das1 needs t wo Dat aRelat ion obj ect s. One Dat aRelat ion obj ect expr esses t he r elat ionship bet ween t he Cust om ers and Orders dat a t ables. A second Dat aRelat ion obj ect represent s t he r elat ionship bet ween t he Orders and Order Det ails dat a t ables. The procedur e builds t he first Dat aRelat ion obj ect by inv ok ing t he Add m et hod for t he Relat ions collect ion of t he das1 dat a set . The first argum ent , w hich is a st r ing w it h t he value Cust Orders , nam es t he Dat aRelat ion obj ect . The next t w o argum ent s ident ify t he colum ns used t o j oin t he t w o dat a t ables. By set t ing t he Nest ed propert y for t he Dat aRelat ion obj ect t o Tr ue, you cause t he XML docum ent t o show orders nest ed wit hin cust om ers. The default value for t he Nest ed propert y is False. I n t his case, t he Wr it eXm l m et hod shows t wo set s of colum n values wit hout any nest ing of colum n v alues fr om one dat a t able w it hin t hose of anot her dat a t able. By invok ing t he Add m et hod a second t im e for t he Relat ions collect ion in t he das1 dat a set , t he pr ocedur e cr eat es a second dat a r elat ionship expr essing t he parent - child st r uct ure bet w een t he Orders and Order Det ails dat a t ables. Finally t he Cr eat eThr eeTierDat aSet pr ocedur e concludes by inv ok ing t he Ret urn st at em ent t o pass t he das1 dat a set back t o t he procedure t hat called it . Sub SaveThreeTierDasAsXmlDocument() Declare and instantiate the das1 data set and populate it with the return data set from the CreateThreeTierDataSet function procedure. Dim das1 As New DataSet() das1 = CreateThreeTierDataSet() Declare string for filename to hold file stream based on XmlTextWriter with contents of das1 data set. Dim str1 As String = _ "c:\SQL Server Development with VBDotNet\" & _ "12\myCustomersSchema.xml" Dim fst1 As New System.IO.FileStream _ (str1, System.IO.FileMode.Create) Dim txw1 As New System.Xml.XmlTextWriter _ (fst1, System.Text.Encoding.Unicode) Write from das1 the XML along with schema. das1.WriteXml(txw1, XmlWriteMode.WriteSchema) Close TextWriter and FileStream. txw1.Close() fst1.Close() End Sub Function CreateThreeTierDataSet() Open connection to northwind database.
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Dim cnn1 As SqlConnection = _ New SqlConnection( _ "Data Source=localhost;" & _ "Initial Catalog=northwind;" & _ "Integrated Security=SSPI") cnn1.Open() Declare and instantiate a data set (das1) Dim das1 As DataSet = New DataSet("CustomerOrders") Declare and instantiate a data adapter (dap1) to fill the Customers data table in das1. Dim dap1 As SqlDataAdapter = _ New SqlDataAdapter( _ "SELECT CustomerID, CompanyName, ContactName, Phone " & _ "FROM Customers", cnn1) dap1.Fill(das1, "Customers") Re-use dap1 to fill the Orders data table in das1. dap1.SelectCommand.CommandText = _ "SELECT OrderID, OrderDate, CustomerID FROM Orders" dap1.Fill(das1, "Orders") Re-use dap1 to fill the OrderDetails data table in das1. dap1.SelectCommand.CommandText = _ "SELECT * FROM [Order Details]" dap1.Fill(das1, "OrderDetails") Close the connection. cnn1.Close() Specify a relationship between Customers and Orders data tables with orders elements nesting within customers elements. das1.Relations.Add("CustOrders", _ das1.Tables("Customers").Columns("CustomerID"), _ das1.Tables("Orders").Columns("CustomerID")). _ Nested = True Specify a relationship between Orders and OrderDetails data tables with OrderDetails elements nesting within orders elements. das1.Relations.Add("OrderDetail", _ das1.Tables("Orders").Columns("OrderID"), _ das1.Tables("OrderDetails").Columns("OrderID"), _ False).Nested = True Return das1 End Function When t he SaveThr eeTierDasAsXm lDocum ent pr ocedur e invokes t he Wr ileXxm l m et hod w it h it s second argum ent equal t o Xm lWr it eMode.Wr it eSchem a, t he m et hod act ually wr it es t wo docum ent s in one. The XSD schem a for t he XML argum ent appears befor e t he act ual dat a. The .NET docum ent at ion refers t o t his kind of schem a as an inline schem a because it appears in line w it h t he XML dat a t hat follows it . The schem a for t he XML docum ent cor responding t o das1 is reasonably com plex because it specifies colum ns from t hree t ables, t wo dat a relat ionship specificat ions, and support ing elem ent s, such as const raint s t o enable t he Dat aRelat ion obj ect s. Figur es 12- 6 and 12- 7 show port ions of t he
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schem a in br owser w indows; t he schem a is t oo long t o fit in one w indow. This schem a appears at t he beginning of t he XML docum ent nam ed in t he SaveThreeTierDasAsXm lDocum ent procedure. The XML docum ent s filenam e is m y Cust om ersSchem a.x m l in t he c: \ SQL Ser ver Dev elopm ent w it h VBDot Net \ Chapt er12 folder. I n t est ing t he applicat ion on your syst em , you m ay care t o change t he dest inat ion folder for t he XML docum ent t o a folder t hat y ou alr eady have on your w orkst at ion. Figu re 1 2 - 6 . Th e first p a rt of t h e in lin e sche m a for t h e X M L d ocum e n t in t h e m yCu st om e rsSch e m a. xm l file .
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Figu r e 1 2 - 7 . Th e se con d pa rt of t h e inlin e sch em a for t h e X M L docu m e n t in t h e m yCu st om er sSch em a .x m l file .
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As y ou can see fr om t he schem a s lengt h and com plex it y, it is of value t o be able t o wr it e t he schem a aut om at ically. Cr eat ing a dat a set in code should be fair ly st raight forward by t his point in t he book. I n any event , if y ou ar e building solut ions wit h ADO.NET, it is highly likely t hat y ou w ill gain a com fort level wit h building dat a set s pr ogr am m at ically . Therefore, using a pr ogram m at ically cr eat ed dat a set as t he basis for a schem a m ay be a useful pr ocess if you ar en t handy at specify ing XSD schem as fr om scrat ch. I n fact , writ ing out t he schem as and corr elat ing t hem w it h t he design of y our dat a set s m ay be a way t o hav e Visual Basic .NET t each you XSD sy nt ax so t hat you can ev ent ually wr it e y our ow n com plex schem as from scrat ch. Figure 12- 8 shows an excerpt from t he beginning of t he XML dat a in m y Cust om ersSchem a.xm l. You can see all of t he first order ( OrderI D 10643) and t he beginning of t he second order ( OrderI D 10692) for t he cust om er w it h a Cust om erI D value of ALFKI . Not ice how orders nest wit hin cust om ers. Also, t he line it em s, or order det ails, for an order nest w it hin an order . Figu r e 1 2 - 8 . An e x cer pt fr om t h e b egin nin g of t h e X M L d at a in t he m yCu st om e r sSch em a .x m l file .
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