Beginning C# 2008 Databases From Novice to Professional phần 7 pot

Listing 13-4.ModifyDataTable.csusing System;using System.Data; using System.Data.SqlClient; namespace Chapter13{ class ModifyDataTable{ static void Mainstring[] args{ // connection strin

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// create data viewDataView dv = new DataView(

dt,

"country = 'Germany'",

"country",DataViewRowState.CurrentRows);

// display data from data viewforeach (DataRowView drv in dv){

for (int i = 0; i < dv.Table.Columns.Count; i++)Console.Write(drv[i] + "\t");

Console.WriteLine();

}}catch(Exception e){

Console.WriteLine("Error: " + e);

}finally{// close connectionconn.Close();

}}}}

see the results in Figure 13-5

Figure 13-5.Using a data view

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How It Works

This program is basically the same as the other examples, so we’ll focus on its use of adata view You create a new data view and initialize it by passing four parameters to itsconstructor

// create data view

DataView dv = new DataView(

dt,

"country = 'Germany'",

"country",DataViewRowState.CurrentRows);

The first parameter is a data table, the second is a filter for the contents of the datatable, the third is the sort column, and the fourth specifies the types of rows to include

in the data view

System.Data.DataViewRowStateis an enumeration of states that rows can have in adata view’s underlying data table Table 13-1 summarizes the states

Table 13-1.Data View Row States

DataViewRowState Members Description

CurrentRows Current rows including unchanged, new, and modified ones

ModifiedCurrent The current version of a modified row

ModifiedOriginal The original version of a modified row

OriginalRows Original rows, including unchanged and deleted rows

Unchanged A row that hasn’t been modified

Every time a row is added, modified, or deleted, its row state changes to the priate one in Table 13-1 This is useful if you’re interested in retrieving, sorting, or filteringspecific rows based on their state (for example, all new rows in the data table or all rowsthat have been modified)

appro-You then loop through the rows in the data view

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// display data from data view

Just as a data row represents a single row in a data table, a data row view (perhaps

it would have been better to call it a data view row) represents a single row in a data

view You retrieve the filtered and the sorted column data for each data row view and

output it to the console

As this simple example suggests, data views offer a powerful and flexible means ofdynamically changing what data one works within a data table

Modifying Data in a Dataset

In the following sections, you’ll work through a practical example showing a number of

ways to update data in data tables programmatically Note that here you’ll just modify the

data in the dataset but not update the data in the database You’ll see in the “Propagating

Changes to a Data Source” section how to persist the original data source changes made

to a dataset

■ Note Changes you make to a dataset aren’t automatically propagated to a database To save the

changes in a database, you need to connect to the database again and explicitly perform the necessary

updates

Try It Out: Modifying a Data Table in a Dataset

Let’s update a row and add a row in a data table

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Listing 13-4.ModifyDataTable.csusing System;

using System.Data;

using System.Data.SqlClient;

namespace Chapter13{

class ModifyDataTable{

static void Main(string[] args){

// connection stringstring connString = @"

select

*fromemployeeswherecountry = 'UK'

";

// create connectionSqlConnection conn = new SqlConnection(connString);try

{// create data adapterSqlDataAdapter da = new SqlDataAdapter();da.SelectCommand = new SqlCommand(sql, conn);// create and fill dataset

DataSet ds = new DataSet();

da.Fill(ds, "employees");

// get data table referenceDataTable dt = ds.Tables["employees"];

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// FirstName column should be nullabledt.Columns["firstname"].AllowDBNull = true;

// modify city in first rowdt.Rows[0]["city"] = "Wilmington";

// add a rowDataRow newRow = dt.NewRow();

Console.WriteLine(

"{0} {1} {2}",row["firstname"].ToString().PadRight(15),row["lastname"].ToString().PadLeft(25),row["city"]);

}}}}

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3. Make ModifyDataTable the startup project, and run it by pressing Ctrl+F5 Youshould see the results in Figure 13-6.

Figure 13-6.Modifying a data table

How It Works

As before, you use a single data table in a dataset

// get data table reference

DataTable dt = ds.Tables["employees"];

Next, you can see an example of how you can change the schema information You

// FirstName column should be nullable

dt.Columns["firstname"].AllowDBNull = true;

the position of a column may change if the database table schema changes

You can modify a row using the same technique You simply select the appropriaterow and set its columns to whatever values you want, consistent with the column datatypes, of course The following line shows the City column of the first row of the datasetbeing changed to Wilmington

// modify City in first row

dt.Rows[0]["city"] = "Wilmington";

Next you add a new row to the data table

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you were to persist the changes to the database, SQL Server would automatically provide

a value for it

Updating data sources requires learning more about data adapter methods andproperties Let’s take a look at these now

Propagating Changes to a Data Source

You’ve seen how a data adapter populates a dataset’s data tables What you haven’t looked

at yet is how a data adapter updates and synchronizes a data source with data from a

which supports queries)

• UpdateCommand

• InsertCommand

• DeleteCommandWe’ll describe each of these properties briefly and then put them to work

UpdateCommand Property

The UpdateCommandproperty The UpdateCommandproperty of the data adapter holds the

called

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For example, to update the City column in the Employees table with the data from

adapter, dtis the data table, connis the connection, and dsis the dataset):

// create command to update Employees City column

da.UpdateCommand = new SqlCommand(

two data column values for the first row in a data table in it It’s valid SQL, but that’s itsonly virtue and it’s not much of one, since it updates only one database row, the row inEmployees corresponding to the first data row in the employees data table

Another approach works for any number of rows Recall from the Command

statements You can use them in any query or data manipulation statement Let’s recodethe preceding code with command parameters

Try It Out: Propagating Dataset Changes to a Data Source

Here you’ll change the city in the first row of the Employees table and persist the change

in the database

inser-tion logic removed since they’re irrelevant here.)

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Listing 13-5.PersistChanges.csusing System;

using System.Data;

class PersistChanges{

select

";

// SQL to update employeesstring upd = @"

update employeesset

city = @citywhere

employeeid = @employeeid

";

// create connectionSqlConnection conn = new SqlConnection(connString);

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try{// create data adapterSqlDataAdapter da = new SqlDataAdapter();da.SelectCommand = new SqlCommand(qry, conn);// create and fill dataset

Console.WriteLine(

}// update Employees//

// create commandSqlCommand cmd = new SqlCommand(upd, conn);//

// map parameters//

// Citycmd.Parameters.Add(

"@city",SqlDbType.NVarChar,15,

"city");

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// EmployeeIDSqlParameter parm =cmd.Parameters.Add(

"@employeeid",SqlDbType.Int,4,

"employeeid");

parm.SourceVersion = DataRowVersion.Original;

//

// update databaseda.UpdateCommand = cmd;

da.Update(ds, "employees");

}catch(Exception e){

}}}}

should see the result in Figure 13-7

Figure 13-7.Modifying a row

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How It Works

from sqlto updin order to clearly distinguish it from this statement

// SQL to update employees

string upd = @"

update employeesset

city = @citywhere

";

at it piece by piece Creating a command is nothing new, but notice that you use the

// update Employees

//

// create command

SqlCommand cmd = new SqlCommand(upd, conn);

a data column named city Note that you don’t specify the data table, but you must besure the type and length are compatible with this column in whatever data table youeventually use

// city

cmd.Parameters.Add(

"city");

changes Although it doesn’t really matter here, since you don’t change any employee IDs,it’s a good habit to specify the original version for primary keys, so if they do change, thecorrect rows are accessed in the database table Note also that you save the reference

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// EmployeeID

SqlParameter parm =

cmd.Parameters.Add(

"employeeid");

parm.SourceVersion = DataRowVersion.Original;

update the Employees table so it will be the SQL the data adapter executes when you call

its Updatemethod You then call Updateon the data adapter to propagate the change to

the database Here you have only one change, but since the SQL is parameterized, the

data adapter will look for all changed rows in the employees data table and submit

updates for all of them to the database

// Update database

da.UpdateCommand = cmd;

Figure 13-7 shows the change to the city, and if you check with Database Explorer

or the SSMSE, you’ll see the update has been propagated to the database The city for

employee Steven Buchanan is now Wilmington, not London

InsertCommand Property

propagated to the database

Try It Out: Propagating New Dataset Rows to a Data Source

in Listing 13-4

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Listing 13-6.PersistAdds.csusing System;

using System.Data;

class PersistAdds{

server = \sqlexpress;integrated security = true;database = northwind

";

// querystring qry = @"

select

";

// SQL to insert employeesstring ins = @"

insert into employees(

firstname,lastname,titleofcourtesy,city,

country)

values

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try{// create data adapterSqlDataAdapter da = new SqlDataAdapter();

da.SelectCommand = new SqlCommand(qry, conn);

// create and fill datasetDataSet ds = new DataSet();

Console.WriteLine(

}

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// insert employees//

// create commandSqlCommand cmd = new SqlCommand(ins, conn);//

// map parameterscmd.Parameters.Add(

"@firstname",SqlDbType.NVarChar,10,

"firstname");

cmd.Parameters.Add(

"@lastname",SqlDbType.NVarChar,20,

"lastname");

cmd.Parameters.Add(

"@titleofcourtesy",SqlDbType.NVarChar,25,

"titleofcourtesy");

cmd.Parameters.Add(

"city");

cmd.Parameters.Add(

"@country",SqlDbType.NVarChar,15,

"country");

//

// insert employeesda.InsertCommand = cmd;

}finally

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{// close connectionconn.Close();

}}}}

see the results in Figure 13-8

Figure 13-8.Adding a row

How It Works

from sqlto insin order to clearly distinguish it from this statement

country)

values(

";

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You replace the updatecomment in the tryblock with quite a bit of code Let’s look at

it piece by piece Creating a command is nothing new, but notice that you use the insertSQL variable (ins), not the query one (sql)

// insert employees

//

// create command

SqlCommand cmd = new SqlCommand(ins, conn);

Then you configure the command parameters The five columns for which you’llprovide values are each mapped to a named command parameter You don’t supply theprimary key value since it’s generated by SQL Server, and the other columns are nullable,

so you don’t have to provide values for them Note that all the values are current values,

// map parameters

cmd.Parameters.Add(

"@firstname",SqlDbType.NVarChar,10,

"firstname");

cmd.Parameters.Add(

"@lastname",SqlDbType.NVarChar,20,

"lastname");

cmd.Parameters.Add(

"@titleofcourtesy",SqlDbType.NVarChar,25,

"titleofcourtesy");

cmd.Parameters.Add(

"city");

cmd.Parameters.Add(

"@country",SqlDbType.NVarChar,15,

"country");

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Finally, you set the data adapter’s InsertCommandproperty with the command toinsert into the Employees table so it will be the SQL the data adapter executes when you

to the database Here you add only one row, but since the SQL is parameterized, the data

adapter will look for all new rows in the employees data table and submit inserts for all

of them to the database

Try It Out: Propagating New Dataset Rows to a Data Source

the database

Listing 13-7.PersistDeletes.csusing System;

using System.Data;

class PersistDeletes{

static void Main(string[] args)

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{// connection stringstring connString = @"

select

";

// SQL to delete employeesstring del = @"

delete from employeeswhere

";

try{// create data adapterSqlDataAdapter da = new SqlDataAdapter();da.SelectCommand = new SqlCommand(qry, conn);// create and fill dataset

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// delete employees//

// create commandSqlCommand cmd = new SqlCommand(del, conn);

//

// map parameterscmd.Parameters.Add(

"employeeid");

//

// select employeesstring filt = @"

firstname = 'Roy'and

lastname = 'Beatty'

";

//

// delete employeesforeach (DataRow row in dt.Select(filt)){

row.Delete();

}da.DeleteCommand = cmd;

// display rowsforeach (DataRow row in dt.Rows){

Console.WriteLine(

}}catch(Exception e)

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{Console.WriteLine("Error: " + e);

}}}}

see the output in Figure 13-9

Figure 13-9.Deleting a row

How It Works

from sqlto delin order to clearly distinguish it from this statement)

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// map parameters

cmd.Parameters.Add(

"employeeid");

you select the row to delete and delete it Actually, you select all rows for employees

named Roy Beatty, since you don’t know (or care about) their employee IDs Although

you expect only one row to be selected, you use a loop to delete all the rows (If you were

to run the PersistAdds program multiple times, you’d have more than one row that

matches this selection criteria.)

// select employees

string filt = @"

firstname = 'Roy'and

delete from the Employees table so it will be the SQL the data adapter executes when

changes to the database

da.DeleteCommand = cmd;

Whether you delete one row or several, your SQL is parameterized, and the dataadapter will look for all deleted rows in the employees data table and submit deletes for

all of them to the Employees database table

If you check with Database Explorer or the SSMSE, you’ll see the row has beenremoved from the database Sir Roy Beatty is no longer in the Employees table

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Command Builders

Although it’s straightforward, it’s a bit of a hassle to code SQL statements for the

UpdateCommand,InsertCommand, and DeleteCommandproperties, so each data provider has

its own command builder If a data table corresponds to a single database table, you

InsertCommand, and DeleteCommandproperties for a data adapter This is all done

Updatemethod, you should call the RefreshSchemamethod on the command builder torefresh the metadata accordingly

To create a command builder, you create an instance of the data provider’s commandbuilder class, passing a data adapter to its constructor For example, the following codecreates a SQL Server command builder:

SqlDataAdapter da = new SqlDataAdapter();

SqlCommandBuilder cb = new SqlCommandBuilder(da);

■ Note For a command builder to work, the SelectCommanddata adapter property must contain aquery that returns either a primary key or a unique key for the database table If none is present, an

InvalidOperationexception is generated, and the commands aren’t generated

Try It Out: Using SqlCommandBuilder

Listing 13-8.PersistAddsBuilder.csusing System;

using System.Data;

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class PersistAddsBuilder{

select

";

try{// create data adapterSqlDataAdapter da = new SqlDataAdapter();

da.SelectCommand = new SqlCommand(qry, conn);

// create command builderSqlCommandBuilder cb = new SqlCommandBuilder(da);

// create and fill datasetDataSet ds = new DataSet();

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Console.WriteLine(

}// insert employeesda.Update(ds, "employees");

}}}}

should see the results in Figure 13-10 Roy Beatty is back in the Employees table

Figure 13-10.Adding a row using a command builder

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