Microsoft SQL Server 2005 Developer’s Guide- P22 pptx

Chapter 6: Developing Database Applications with ADO.NET 189
A string variable containing the server name is passed in to the beginning of this
subroutine, and then two SqlConnection objects, cn and cn2, are created that have
identical connection strings. In both cases, the ConnectionString property uses
the SERVER keyword to identify the SQL Server instance to connect to and the
INTEGRATED SECURITY keyword to specify that Windows integrated security
will be used.
After the two SqlConnection objects have been created, a Try-Catch loop is
used to open the connection to SQL Server and capture any run-time errors. Since
the values of these connection strings are identical, they will both be part of the
same connection pool. If the connection strings were different in any way, then two
separate connection pools would have been created. After the connections have been
established, the Close method is used to close each connection.
Pooling Related Connection String Keywords
While the .NET Framework Data Provider for SQL Server automatically handles
connection pooling for you, there are still several connection string keywords that you
can use to alter the SQLConnection object’s connection pooling behavior. Table 6-2
presents the ConnectionString values you can use to customize the SQL Server .NET
Data Provider’s connection pooling behavior.
Name Description
Connection Lifetime After a connection is closed, it’s returned to the pool. Then its creation time is
compared with the current time and the connection is destroyed if the difference
exceeds the value specified by Connection Lifetime. A value of 0 specifies that
pooled connections will have the maximum lifespan.
Connection Reset When ‘True’, this specifies that the connection is reset when it’s removed from the
pool. For Microsoft SQL Server version 7.0, you can set this value to ‘False’ to avoid
an additional server round trip after opening a connection. However, the previous
connection state and database context will not be reset.
Enlist When this value is ‘True’, the connection is automatically created in the current
transaction context of the creation thread if a transaction context exists.
Max Pool Size Specifies the maximum number of connections allowed in the pool.

Min Pool Size Specifies the minimum number of connections maintained in the pool.
Pooling When this value is ‘True’, connection pooling is automatically enabled. ‘False’ allows
you to turn off connection pooling.
Table 6-2 Pooling-Related Connection String Keywords
190 Microsoft SQL Server 2005 Developer’s Guide
NOTE
For those connection string keywords that contain spaces, the spaces are a required part of the
keyword.
Using the SqlCommand Object
Executing dynamic SQL statements and stored procedures are two of the most
common database actions that are required by an application. Dynamic SQL
statements are SQL statements that are read by the database server and executed
when they are sent to the database server from the client application. When the
database receives these SQL statements, they are first parsed to ensure that their
syntax is correct, and then the database engine creates an access plan—essentially
determining the best way to process the SQL statement—and then executes the
statements. Unlike dynamic SQL statements, which are often used for executing
SQL DML operations like creating tables or for data access operations like
performing ad hoc queries, stored procedures are typically used to perform
predefined queries and database update operations. Stored procedures form the
backbone of most database applications. The primary difference between dynamic
SQL statements and stored procedures is that stored procedures are typically created
before the application is executed and reside in the database itself. This gives stored
procedures a significant performance advantage over dynamic SQL statements
because the jobs of parsing the SQL statement and creating the data access plan
have already been completed. It’s worth noting that changes made to data contained
in an ADO.NET DataSet can be posted back to the database using dynamic SQL
statements created by the SqlCommandBuilder class, or else they can be written
back to the database using stored procedures. However, you don’t need to use
the DataSet and DataAdapter in order to update the database. In cases where you

don’t need the data binding and navigation functions provided by the DataSet, the
Command objects can provide a much lighter-weight and more efficient method of
updating the database. In the next sections, you’ll see how to use the SqlCommand
object to execute an ad hoc query, then to execute a SQL DDL statement to build
a table on the target database, followed by two examples using the stored
procedure. The first stored procedure example illustrates passing parameters to a
stored procedure, and the second example illustrates executing a stored procedure
that supplies a return value.
Table 6-3 lists all of the different SQL command execution methods supported by
both the SqlCommand object and the OleDbCommand object.
Chapter 6: Developing Database Applications with ADO.NET 191
Method Description
ExecuteNonQuery The ExecuteNonQuery method is used to execute a SQL statement on
the connected data source. It is used for DDL statements; action queries
like Insert, Update, and Delete operations; as well as ad hoc queries. The
number of rows affected is returned, but no output parameters or result
sets are returned.
ExecuteReader The ExecuteReader method is used to execute a SQL Select statement on
the data source. A fast forward–only result is returned.
ExecuteScalar The ExecuteScalar method is used to execute a stored procedure or
a SQL statement that returns a single scalar value. The first row of the
first column of the result set is returned to the calling application. Any
other returned values are ignored.
ExecuteXMLReader The ExecuteXMLReader method is used to execute a FOR XML SELECT
statement that returns an XML data stream from the data source. The
ExecuteXMLReader command is compatible only with SQL Server 2000
and later.
Executing Dynamic SQL Statements
Dynamic SQL provide an extremely flexible mechanism for working with the
database. Dynamic SQL allows you to execute ad hoc queries and return the results

from action queries, as well as executing SQL DDL statements to create database
objects. The following SQLCommandNonQuery subroutine provides an example
illustrating how you can use dynamic SQL with the ADO.NET SqlCommand object
to check for the existence of a table and conditionally create it if it doesn’t exist:
Private Sub SQLCommandNonQuery(cn As SqlConnection)
Dim sSQL As String = ""
Dim cmd As New SqlCommand(sSQL, cn)
Try
' First drop the table
sSQL = "IF EXISTS " _
& "(SELECT * FROM dbo.sysobjects WHERE id = " _
& "object_id(N’[Department]’) " _
& "AND OBJECTPROPERTY(id, N’IsUserTable’) = 1) " _
& "DROP TABLE [department]"
cmd.CommandText = sSQL
Table 6-3 SqlCommand SQL Statement Execution Methods
192 Microsoft SQL Server 2005 Developer’s Guide
cmd.ExecuteNonQuery()
' Then create the table
sSQL = "CREATE TABLE Department " _
& "(DepartmentID Int NOT NULL, " _
& "DepartmentName Char(25), PRIMARY KEY(DepartmentID))"
cmd.CommandText = sSQL
cmd.ExecuteNonQuery()
Catch e As Exception
MsgBox(e.Message)
End Try
End Sub
In the first part of the SQLCommandNonQuery subroutine, you can see where the
SQL Server connection object is passed as a parameter. The sSQL variable that will

be used to contain the dynamic SQL statements and an instance of the SqlCommand
object named cmd are instantiated. In this example, the constructor of the cmd
SqlCommand object uses two parameters—the first being a string containing the
SQL statement that will be executed and the second being the SqlConnection object
that will provide the connection to the target database server. Here the sSQL string is
initially empty. Next, a Try-Catch structure is set up to execute the SQL commands.
The first action that you can see within the Try-Catch block assigns a SQL statement
to the sSQL variable that checks for the existence of the department table. In this
SQL statement, you can see that a SELECT statement queries the SQL Server
sysobjects table to determine if a User Table named Department exists. If the
Department table is found, a DROP TABLE statement will be executed to remove
the table from the target database. Otherwise, if the Department table isn’t found,
no further action will be taken. In order to actually execute the SQL statement, that
value in the sSQL variable is then assigned to the CommandText property of the
cmd object, and then the ExcuteNonQuery method of the cmd SqlCommand object
is used to send the command to the SQL Server system. The ExecuteNonQuery
method is used to execute a SQL statement that doesn’t return a result set or a specific
return value.
After the first DROP TABLE SQL command has been issued, the same sequence
is followed to execute a Create Table command. First the sSQL variable is assigned a
SQL CREATE TABLE statement that creates a table named Department that consists
of two columns. The first column is an integer data type named DepartmentID,
which is also the primary key, and the second column is a 25-character data type
named DepartmentName. Then the value in the sSQL variable is copied to the cmd
object’s CommandText property, and the ExecuteNonQuery method is called to
execute the CREATE TABLE SQL statement. Following the successful completion
Chapter 6: Developing Database Applications with ADO.NET 193
of the ExecuteNonQuery method, the Department Table will exist in the database
that was earlier identified in the sDB variable.
If an error occurs during any of the operations contained in the Try block, the

code in the Catch block will be executed, and a message box will be displayed
showing the text of the exception condition.
Executing Parameterized SQL Statements
In addition to executing dynamic SQL statements, the SqlCommand object can
also be used to execute stored procedures and parameterized SQL statements. The
primary difference between dynamic SQL and prepared SQL is that dynamic SQL
statements must be parsed and an access plan must be created before each run.
(Technically, some database systems like SQL Server are very smart about the way
this is handled, and they will actually store dynamic statements for a period of time.
Then when the statement is subsequently executed, the existing access plan will be
used. Even so, this depends on the database activity, and with dynamic SQL there’s
no guarantee that the plan will be immediately available.) You can think of prepared
SQL statements as sort of a cross between stored procedures and dynamic SQL.
Like stored procedures, they can accept different parameter values at run time. Like
dynamic SQL, they are not persistent in the database. The SQL statement is parsed,
and the access plan is created when the application executes the SQL statements.
However, unlike dynamic SQL, the prepared SQL is parsed and the access plan
is created only once, when the statement is first prepared. Subsequent statement
execution takes advantage of the existing access plan. The access plan will typically
remain in the procedure cache until the connection is terminated. The following
example shows how to create and execute a prepared SQL statement using the ADO.
NET SqlCommand object:
Private Sub SQLCommandPreparedSQL(cn As SqlConnection)
' Set up the Command object's parameter types
Dim cmd As New SqlCommand("INSERT INTO department VALUES" & _
"(@DepartmentID, @DepartmentName)", cn)
Dim parmDepartmentID = _
New SqlParameter("@DepartmentID", SqlDbType.Int)
parmDepartmentID.Direction = ParameterDirection.Input
Dim parmDepartmentName = _

New SqlParameter("@DepartmentName", SqlDbType.Char, 25)
parmDepartmentName.Direction = ParameterDirection.Input
' Add the parameter objects to the cmd Parameter’s collection
cmd.Parameters.Add(parmDepartmentID)
cmd.Parameters.Add(parmDepartmentName)
194 Microsoft SQL Server 2005 Developer’s Guide
Try
cmd.Prepare()
' Execute the prepared SQL statement to insert 10 rows
Dim i As Integer
For i = 0 To 10
parmDepartmentID.Value = i
parmDepartmentName.Value = "New Department " & CStr(i)
cmd.ExecuteNonQuery()
Next
Catch e As Exception
MsgBox(e.Message)
End Try
End Sub
At the top of the CommandPrepareSQL subroutine, you can see where the
SqlConnection object named cn is passed in, followed by the creation of a new
SqlCommand object named cmd. In this example, the constructor takes two
arguments. The first argument is used to assign a SQL statement to the cmd object.
This can either be a SQL statement or the name of a stored procedure. Here, the
SQL statement is an INSERT statement that adds the values of two columns to the
Department table.
NOTE
The Department table was created in the earlier section of this chapter.
The important point to note in this example is the format of the parameter markers
that are used in the SQL statement. Parameter markers are used to indicate the

replaceable characters in a prepared SQL statement. At run time, these parameters
will be replaced with the actual values that are supplied by the SqlCommand object’s
Parameters collection. Unlike ADO, which uses the question mark character (?) to
indicate replaceable parameters, the SqlCommand object requires that all parameter
markers begin with the @ symbol. This example shows two parameter markers:
@DepartmentID and @DepartmentName. The second argument of the SqlCommand
constructor associates the cmd SqlCommand object with the cn SqlConnection
object that was passed in earlier.
Next, you can see where two SqlParameter objects are created. The first parameter
object, named parmDepartmentID, will be used to supply values to the first parameter
marker (@DepartmentID). Likewise, the second parameter object, named
parmDepartmentName, will supply the values used by the second replaceable
parameter (@DepartmentName). The code example used in this subroutine shows
Chapter 6: Developing Database Applications with ADO.NET 195
three parameters being passed to the SqlParameter’s constructor. The first parameter
supplies the parameter name. Here you need to make sure that the name supplied
to the SqlParameter object’s constructor matches the name that was used in the
parameter marker of the prepared SQL statement. The second parameter that’s
passed to this overloaded version of the SqlParameter constructor specifies the
parameter’s data type.
Here the Direction property is set to input using the ParameterDirection.Input
enumeration. Table 6-4 lists the valid enumerations for the SqlParameter Direction
property.
After the SqlParameter objects have been created, the next step is to add them to
the SqlCommand object’s Parameters collection. In the previous listings, you can see
that you use the Add method of the SqlCommand object’s Parameters collection to
add both the parmDepartmentID and parmDepartmentName SqlParameter objects to
the cmd SqlCommand object. The order in which you add the SqlParameter objects
isn’t important. Next, within the Try-Catch block the Prepare statement is used
to prepare the statement. Note that the Prepare method is executed after all of the

parameter attributes have been described.
NOTE
Using the Prepare operation provides an important performance benefit for parameterized queries
because it instructs SQL Server to issue an sp_prepare statement, thereby ensuring that the
statement will be in the Procedure cache until the statement handle is closed.
Next a For-Next loop is used to add ten rows to the newly created Department table.
Within the For-Next loop, the Value property of each parameter object is assigned
a new data value. For simplicity, the parmDepartmentID parameter is assigned the
value of the loop counter contained in the variable i, while the parmDepartmentName
parameter is assigned a string containing the literal “New Department” along with the
current value of the loop counter. Finally, the SqlCommand object’s ExecuteNonQuery
method is used to execute the SQL statement. In this case, ExecuteNonQuery was
Enumeration Description
ParameterDirection.Input The parameter is an input parameter.
ParameterDirection.InputOutput The parameter is capable of both input and output.
ParameterDirection.Output The parameter is an output parameter.
ParameterDirection.ReturnValue The parameter represents a return value.
Table 6-4 SqlParameterDirection Enumeration
196 Microsoft SQL Server 2005 Developer’s Guide
used because this example is using a SQL action query that doesn’t return any values.
From the SQL Server perspective, running the ExecuteNonQuery method results in the
server issuing an sp_execute command to actually perform the insert.
NOTE
If you need to pass a null value as a parameter, you need to set the parameter to the value
DBNull.Value.
If an error occurs during any of these operations, the code in the Catch block will
be executed and a message box will be displayed showing the text of the exception
condition.
Executing Stored Procedures with Return Values
Stored procedures are the core of most database applications—and for good reason.

In addition to their performance benefits, stored procedures can also be a mechanism
for restricting data access to the predefined interfaces that are exposed by the stored
procedures. Similar to prepared SQL statements, stored procedures get significant
performance benefits from the fact that they are compiled before they are used. This
allows the database to forgo the typical parsing steps that are required as well as
skipping the need to create an access plan. Stored procedures are the true workhorse
of most database applications, and they are almost always used for database insert,
update, and delete operations, as well as for retrieving single values and results sets.
In the following examples, you see how to execute SQL Server stored procedures
using the SqlCommand object. In the first example that follows, you’ll see how to
execute a stored procedure that accepts a single input parameter and returns a scalar
value.
The following listing presents the T-SQL source code required to create the CostDiff
stored procedure that will be added to the sample AdventureWorks database. You can
create this stored procedure by executing this code using SQL Server Management
Studio.
CREATE PROCEDURE CostDiff
@ProductID int
AS
DECLARE @CostDiff money
SELECT CostDiff = (ListPrice - StandardCost)
FROM Production.Product WHERE ProductID = @ProductID
RETURN @CostDiff
Chapter 6: Developing Database Applications with ADO.NET 197
In this listing, you can see that the CostDiff stored procedure accepts a single input
parameter. That parameter is an Integer value that’s used to identify the ProductID.
The CostDiff stored procedure returns the cost difference of that ProductID from
the Production.Product table in the AdventureWorks database. The cost difference is
calculated by retrieving the ListPrice number and subtracting it from the value in the
StandardCost column. The results are then assigned to the @CostDiff variable, which

is returned as a scalar value by the stored procedure. After the sample stored procedure
has been created in the AdventureWorks database, it can be called by your ADO.NET
applications. The following example shows how to use the SqlCommand class from
VB.NET to execute the CostDiff stored procedure and retrieve the scalar value that
it returns:
Private Sub SQLCommandSPScalar(cn As SqlConnection)
' Create the command object and set the SQL statement
Dim cmd As New SqlCommand("CostDiff", cn)
cmd.CommandType = CommandType.StoredProcedure
' Create the parameter
cmd.Parameters.Add("@ProductID", SqlDbType.Int)
cmd.Parameters("@ProductID").Direction = _
ParameterDirection.Input
cmd.Parameters("@ProductID").Value = 1
Try
Dim nCostDiff As Decimal
nCostDiff = cmd.ExecuteScalar()
' Put to textbox on displayed form
txtMid.Text = nCostDiff
Catch e As Exception
MsgBox(e.Message)
End Try
End Sub
In the beginning of this routine you can see where the cn SqlConnection object
is passed in, followed by the creation of the SqlCommand object named cmd. In
this example, the constructor for the SqlCommand object uses two parameters. The
first parameter is a string that accepts the command that will be executed. This can
be either a SQL statement or the name of the stored procedure. In this example,
you can see that the name of the CostDiff stored procedure is used. The second
parameter is used for the name of the SqlConnection object that will be used to

connect to the target database. After the cmd SqlCommand object has been created,
its CommandType property is set to CommandType.StoredProcedure, indicating that
a stored procedure will be executed. The CommandType property can accept any of
the values shown in Table 6-5.
198 Microsoft SQL Server 2005 Developer’s Guide
CommandType Values Description
CommandType.StoredProcedure The command is a stored procedure.
CommandType.TableDirect The command is the name of a database table.
CommandType.Text The command is a SQL statement.
After the SqlCommand object’s CommandType property is set to CommandType.
StoredProcedure, the SqlParameter object used to supply the input value to the
CostDiff stored procedure is created. SqlParameter objects can be created either by
using the SqlParameter class constructor or by executing the SqlCommand object’s
Parameters collection Add method. In this example, the parameter is created using the
Add method of the SqlCommand object’s Parameters collection. The first parameter
supplied to the Add method is a string containing the name of the parameter, in this
case “@ProductID”. Again, note that replaceable parameters used by the SqlParameter
object must begin with the ampersand symbol (@). The second parameter uses the
SqlDbType.Int enumeration to indicate that the parameter will contain an Integer
value. The next line sets the Direction property to the value ParameterDirection.Input
to indicate that this is an input parameter. Finally, the SqlParameter object’s Value
property is set to 1—storing a value of 1 to pass to the CostDiff stored procedure.
The next section of code sets up a Try-Catch block to execute the CostDiff
stored procedure. The important point to note in the Try-Catch block is that the cmd
SqlCommand object’s ExecuteScalar method is used to execute the CostDiff stored
procedure and the return value is assigned to the nCostDiff variable. The contents of
the nCostDiff variable are then assigned to a text box named txtMid that is defined
on the Windows form for this project. As in earlier examples, if the stored procedure
fails, a message box showing the error text will be displayed to the end user.
Executing Transactions

Transactions enable you to group together multiple operations that can be performed
as a single unit of work, which helps to ensure database integrity. For instance,
transferring funds from your saving account to your checking account involves
multiple database operations, and the transfer cannot be considered complete unless
all of the operations are successfully completed. A typical transfer from your savings
account to your checking account requires two separate but related operations:
a withdrawal from your savings account and a deposit to your checking account.
If either operation fails, the transfer is not completed. Therefore both of these
Table 6-5 CommandType Values