Microsoft Visual Basic Databases - Introduction to SQL Functions

	Introduction to SQL Functions

Functions Fundamentals

Introduction

Many languages support functions, including Visual Basic, C/C++, and Pascal, etc. As seen in the Visual Basic language, a function is a relatively small task that should be performed aside but can be accessed any time to give a result. In Transact-SQL, a function is considered an object. Based on this, you must create a function and execute it before using it. The function then becomes part of a database and it can be accessed like an object.

Practical Learning: Introducing Functions

Start Microsoft Visual Basic and create a Windows Application named TriStateUtilityCompany1

Design the form as follows:

Control	Text	Name	Other Properties
Label	Customer Name
TextBox		txtCustomerName
Label	Counter: ___________
Label	Last Month:
TextBox		txtLastMonth	TextAlign: Right
Label	This Month:
TextBox		txtThisMonth	TextAlign: Right
Label	Consumption:
TextBox		txtConsumption	TextAlign: Right
Button	Evaluate	btnEvaluate
Label	Invoice: ___________
Label	Amount Due:
TextBox		txtAmountDue	TextAlign: Right
Label	Database: _________
Button	Database	btnDatabase
Button	Close	btnClose

Double-click the Database button and implement its event as follows:

Imports System.Data.SqlClient

Public Class Exercise

    Private Sub btnDatabase_Click(ByVal sender As System.Object, _
                                  ByVal e As System.EventArgs) _
                                  Handles btnDatabase.Click
        Dim strConnection As String = _
         "Data Source=(local);Integrated Security=yes"

        Using connection As SqlConnection = _
            	New SqlConnection(strConnection)
            Dim command As SqlCommand = _
         	New SqlCommand("CREATE DATABASE UtilityCompany1;", _
          			connection)

            connection.Open()
            command.ExecuteNonQuery()

            MsgBox("A database named ""UtilityCompany1"" has been created.")
        End Using
    End Sub
End Class

In the Class Name combo box, select btnClose

In the Method Name combo box, select Click and implement the event as follows:

Private Sub btnClose_Click(ByVal sender As Object, _
                               ByVal e As System.EventArgs) _
                               Handles btnClose.Click
        End
End Sub

Execute the application
Click the Database button
Close the form and return to your programming environment

Function Creation Fundamentals

There are various ways you can create a function:

In the Object Explorer, you can expand the desired database, expand the Programmatically node, and expand the Functions node. Right-click Scalar-Valued Function and click New Scalar-Valued Function... Sample code would be generated for you. You can then modify to customize it
Open an empty query window. Display the Templates Explorer and expand the Function node. Drag Create Scalar-Valued Function and drop it in the query window
You can open a new empty query window and start typing your code in it
Programmatically, you can include of a function creation in the command

In Transact-SQL, the primary formula of creating a function is:

CREATE FUNCTION FunctionName()

The Name of a Function

We mentioned already that, in SQL, a function was created as an object. As such, it must have a name. In our lessons, here are the rules we will use to name our functions:

The name of a function will resemble an action. An example is calculate
A name will start with either an underscore or a letter. Examples are _n, act, or Perform
After the first character as an underscore or a letter, the name will have combinations of underscores, letters, and digits. Examples are _n24 or act_52_t
A name will not include special characters such as !, @, #, $, %, ^, &, or *
We will avoid using spaces in a name
If the name is a combination of words, each word will start in uppercase. Examples are DoSomething, _CreateStudentsRecords, Get_Age, or _Calculate_Volume_Area

Returning a Value From a Function

For a function to be useful, it must produce a result. This is also said that the function returns a result or a value. When creating a function, you must specify the type of value that the function would return. To provide this information, after the name of the function, type the RETURNS keyword followed by a definition for a data type. Here is a simple example:

CREATE FUNCTION Addition()
RETURNS Decimal(6,3)

After specifying the type of value that the function would return, you can create a body for the function. The body of a function starts with the BEGIN and ends with the END keywords. Here is an example:

CREATE FUNCTION Addition()
RETURNS Decimal(6,3)
BEGIN

END

Optionally, you can type the AS keyword before the BEGIN keyword:

CREATE FUNCTION Addition()
RETURNS Decimal(6,3)
AS
BEGIN

END

Between the BEGIN and END keywords, which is the section that represents the body of the function, you can define the assignment the function must perform. After performing this assignment, just before the END keyword, you must specify the value that the function returns. This is done by typing the RETURN keyword followed by an expression. A sample formula is:

CREATE FUNCTION Addition()
RETURNS Decimal(6,3)
AS
BEGIN
    RETURN Expression
END

Here is an example

CREATE FUNCTION GetFullName()
RETURNS varchar(100)
AS
BEGIN
	RETURN 'Doe, John'
END

Practical Learning: Creating a Function

Change the code of the Database button as follows:

Private Sub btnDatabase_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles btnDatabase.Click
    Dim strConnection As String = _
  	"Data Source=(local);" & _
  	"Database='UtilityCompany1';" & _
  	"Integrated Security=yes;"
    Dim CreateFunction As String = "CREATE FUNCTION EvaluateInvoice() " & _
    			           "RETURNS Decimal(8, 2) " & _
           			   "AS " & _
           			   "BEGIN " & _
           			   "    RETURN 8.50 " & _
           			   "END;"

    Using connection As SqlConnection = _
	     New SqlConnection(strConnection)
        Dim command As SqlCommand = _
     		New SqlCommand(CreateFunction, connection)

        connection.Open()
        Command.ExecuteNonQuery()
        MsgBox("A function named ""EvaluateInvoice"" has been created.")
    End Using
End Sub

Execute the application
To actually create the function, click the Database button
Close the form and return to your programming environment

Function Calling

After a function has been created, you can use the value it returns. Using a function is also referred to as calling it. To call a function, you must qualify its name. To do this, type the name of the database in which it was created, followed by the period operator, followed by dbo, followed by the period operator, followed by the name of the function, and its parentheses. The formula to use is:

DatabaseName.dbo.FunctionName()

Because a function returns a value, you can use that value as you see fit. For example, you can use either PRINT or SELECT to display the function's value in a query window. Here is an example that calls the above Addition() function:

PRINT Exercise.dbo.GetFullName()

As an alternative, to call a function, in the Object Explorer, right-click its name, position the mouse on Script Function As, SELECT To, and click New Query Editor Window.

Practical Learning: Calling a Function

In the Class Name combo box, select btnEvaluate

In the Method Name combo box, select Click and implement the event as follows:

Private Sub btnEvaluate_Click(ByVal sender As Object, _
                              ByVal e As System.EventArgs) _
                              Handles btnEvaluate.Click
    Dim strConnection As String = _
         "Data Source=(local);" & _
         "Database='UtilityCompany1';" & _
         "Integrated Security=yes;"
    Dim ExecuteFunction As String = "SELECT dbo.EvaluateInvoice();"

    using connection As SqlConnection  = New SqlConnection(strConnection)
        Dim command As SqlCommand = _
	     New SqlCommand(ExecuteFunction, connection)

        connection.Open()
        Dim rdr As SqlDataReader = command.ExecuteReader()

        While rdr.Read()
            txtAmountDue.Text = rdr(0)
        End While

        rdr.Close()
    End Using
End Sub

Execute the application and click the Evaluate button:
Close the form and return to your programming environment

Function Maintenance

Introduction

Because a function in Transact-SQL is treated as an object, it may need maintenance. Some of the actions you would take include renaming, modifying, or deleting a function.

Renaming a Function

If you create a function and execute it, it is stored in the Scalar-Valued Functions node with the name you gave it. If you want, you can change that name but keep the functionality of the function.

To rename a function, in the Object Explorer, right-click it and click Rename. Type the desired new name and press Enter.

Deleting a Function

If you create a function and decide that you do not need it any more, you can delete it.

To delete a function in the Object Explorer, locate the function in the Functions section, right-click it and click Delete. The Delete Object dialog box would come up. If you still want to delete the function, click OK; otherwise, click Cancel.

To programmatically delete a function:

In a query window, type DROP FUNCTION followed by the name of the function and execute the statement
In the Object Explorer, right-click the name of the function, position the mouse on Script Function As, DROP To, and click New Query Editor Window
Open a new query window associated with the database that contains the function. Display the Templates Explorer and expand the Function node. Drag the Drop Function node and drop it in the empty query window

Practical Learning: Deleting a Function

Change the code of the Database button as follows:

Private Sub btnDatabase_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles btnDatabase.Click
    Dim strConnection As String = _
	  	"Data Source=(local);" & _
	  	"Database='UtilityCompany1';" & _
		"Integrated Security=yes;"
    Dim CreateFunction As String = "DROP FUNCTION EvaluateInvoice;"

    Using connection As SqlConnection = New SqlConnection(strConnection)
        Dim command As SqlCommand = _
             New SqlCommand(CreateFunction, connection)

        connection.Open()
        command.ExecuteNonQuery()
        MsgBox("A function named ""EvaluateInvoice"" has been deleted.")
    End Using
End Sub

Execute the application and click the Database button
Close the form and return to your programming environment

Modifying a Function

As mentioned already, in the body of the function, you define what the function is supposed to take care of. As a minimum, a function can return a simple number, typed on the right side of the RETURN keyword. Here is an example:

CREATE FUNCTION Addition()
RETURNS int
BEGIN
    RETURN 1
END

You can also declare new variables in the body of the function to help in carrying the assignment. A variable declared in the body of a function is referred to as a local variable. Once such a variable has been declared, it can be used like any other variable. Here is an example:

CREATE FUNCTION Addition()
RETURNS int
BEGIN
    DECLARE @Number1 int
    SET @Number1 = 588
    RETURN @Number1 + 1450
END

Function Arguments

Introduction

In order to carry its assignment, a function can be provided with some values. Put it another way, when you create a function, instead of, or in addition to, local variables, you may want the code that will call the function to provide the values needed to perform the assignment. For example, imagine you want to create a function that would generate employees email addresses when a user has entered a first and last name. At the time you are creating the function, you cannot know or predict the names of employees, including those who have not even been hired yet. In this case, you can write the whole function but provide one or more placeholders for values that would be supplied when the function is called.

An external value that is provided to a function is called a parameter. A function can also take more than one parameter. Therefore, when you create a function, you also decide whether your function would take one or more parameters and what those parameters, if any, would be.

A Parameterized Function

We have already seen that a function's name is also followed by parentheses. If the function does not use an external value, its parentheses can be left empty. If a function will use an external value, when you create the function, you must specify a name and the type of value of the parameters. The name of the parameter is created with the @ sign, like a variable as we saw in the previous lesson. Here is an example:

CREATE FUNCTION Addition(@Number1 Decimal(6,2))

When a function takes a parameter, in the body of the function, you can use the parameter as if you knew its value, as long as you respect the type of that value. Here is an example:

CREATE FUNCTION Addition(@Number1 Decimal(6,2))
RETURNS Decimal(6,2)
BEGIN
    RETURN @Number1 + 1450
END

Calling a Parameterized Function

When you call a function that takes one parameter, you must supply a value for that argument. To do this, type the value of the parameter in the parentheses of the function. Here is an example:

Function

A Function With Various Arguments

Instead of only one parameter, you can also create a function that takes more than one parameter. In this case, separate the arguments in the parentheses of the function with a comma. Here is an example:

CREATE FUNCTION Addition(@Number1 Decimal(6,2), @Number2 Decimal(6,2))

Once again, in the body of the function, you can use the parameters as if you already knew their value. You can also declare local variables and involve them with parameters as you see fit. Here is an example:

CREATE FUNCTION Addition(@Number1 Decimal(6,2),
			 @Number2 Decimal(6,2))
RETURNS Decimal(6,2)
BEGIN
    DECLARE @Result Decimal(6,2)
    SET @Result = @Number1 + @Number2
    RETURN @Result
END;
GO

When calling a function that takes more than one parameter, in the parentheses of the function, provide a value for each parameter, in the exact order they appear in the parentheses of the function. Here is an example:

PRINT Variables1.dbo.Addition(1450, 228);

You can also pass the names of already declared and initialized variables. Here is an example that calls the above function:

DECLARE @Nbr1 Decimal(6,2),
        @Nbr2 Decimal(6,2)
SET @Nbr1 = 4268.55
SET @Nbr2 =26.83
SELECT @Nbr1 As First,
       @Nbr2 As Second,
       Variables1.dbo.Addition(@Nbr1, @Nbr2) AS Result

This would produce:

Function

Practical Learning: Creating and Calling a Function with Argument

Change the code of the Database button as follows:

Private Sub btnDatabase_Click(ByVal sender As System.Object, _
                                  ByVal e As System.EventArgs) _
                                  Handles btnDatabase.Click
        Dim strConnection As String = _
  "Data Source=(local);" & _
  "Database='UtilityCompany1';" & _
  "Integrated Security=yes;"
        ' The following function is used to calculate the customer's next bill
        ' Some of the empty spaces, not required,
        ' are meant to make the code easier to read
        Dim CreateFunction As String = _
        "CREATE FUNCTION EvaluateInvoice(@Counter int) " & _
           "RETURNS decimal(6, 2) " & _
           "AS " & _
           "BEGIN " & _
           "	 DECLARE @BaseCharge money, " & _
           "	 	 @Counter0To50 int, " & _
           "	 	 @Counter50To150 int, " & _
           "	 	 @Counter150To200 int, " & _
           "	 	 @CounterOver200 int, " & _
           "	  	 @First50 decimal(6, 2), " & _
           "	 	 @FiftyTo150 decimal(6, 2), " & _
           "	 	 @OneFiftyTo200 decimal(6, 2) ," & _
           "	 	 @Over200 decimal(6, 2), " & _
           "	 	 @TotalCharge money; " & _
                    "" & _
           "	 SET @BaseCharge = 8.50; " & _
           "	 SET @Counter0To50 = 0; " & _
           "	 SET @Counter50To150 = 0; " & _
           "	 SET @Counter150To200 = 0; " & _
           "	 SET @CounterOver200 = 0; " & _
           "	 SET @First50 = 0.00; " & _
           "	 SET @FiftyTo150 = 0.00; " & _
           "	 SET @OneFiftyTo200 = 0.00; " & _
           "	 SET @Over200 = 0.00; " & _
           "	 SET @TotalCharge = 0.00; " & _
                    "" & _
           "	 IF @Counter <= 50 " & _
           "	     BEGIN" & _
           "	 	SET @Counter0To50 = @Counter; " & _
           "	 	SET @Counter50To150 = 0; " & _
           "	 	SET @Counter150To200 = 0; " & _
           "	 	SET @CounterOver200 = 0; " & _
           "	     END;" & _
                    "" & _
           "	 IF (@Counter > 50) AND (@Counter <= 150) " & _
           "	     BEGIN" & _
           "	 	SET @Counter0To50 = 50; " & _
           "	 	SET @Counter50To150 = @Counter - 50; " & _
           " 		SET @Counter150To200 = 0; " & _
           " 		SET @CounterOver200 = 0; " & _
           " 	     END;" & _
                    "" & _
           "	 IF (@Counter > 150) AND (@Counter <= 300) " & _
           "	     BEGIN " & _
           "		SET @Counter0To50 = 50; " & _
           " 		SET @Counter50To150 = 100; " & _
           " 		SET @Counter150To200 = @Counter - 150; " & _
           " 		SET @CounterOver200 = 0; " & _
           " 	     END; " & _
                    "" & _
           " 	IF @Counter > 300 " & _
           "        BEGIN " & _
           " 		SET @Counter0To50 = 50; " & _
           " 		SET @Counter50To150 = 100; " & _
             " 		SET @Counter150To200 = 100; " & _
           " 		SET @CounterOver200 = @Counter - 300; " & _
           "        END;" & _
                    "" & _
           " 	SET @First50 = @Counter0To50 * 0.7675; " & _
           " 	SET @FiftyTo150 = @Counter50To150 * 0.6248; " & _
           " 	SET @OneFiftyTo200 = @Counter150To200 * 0.5825; " & _
           " 	SET @Over200 = @CounterOver200 * 0.5037; " & _
                    "" & _
      " SET @TotalCharge = @BaseCharge + @First50 + @FiftyTo150 & " & _
           " 	@OneFiftyTo200 + @Over200; " & _
                "" & _
           " 	RETURN @TotalCharge; " & _
           "END;"

        Using connection As SqlConnection = _
     New SqlConnection(strConnection)
            Dim command As SqlCommand = _
  New SqlCommand(CreateFunction, connection)

            connection.Open()
            Command.ExecuteNonQuery()

            MsgBox("A function named ""EvaluateInvoice"" has been created.")
        End Using
End Sub

Change the code of the Evaluate button as follows:

Private Sub btnEvaluate_Click(ByVal sender As Object, _
                              ByVal e As System.EventArgs) _
                              Handles btnEvaluate.Click
    Dim StartCounter As Integer, EndCounter As Integer
    Dim Consumption As Integer

    Try
        StartCounter = CInt(txtCounterLastMonth.Text)
    Catch
        MsgBox("Invalid Start Counter")
    End Try

    Try
        EndCounter = CInt(txtCounterThisMonth.Text)
    Catch
        MsgBox("Invalid End Counter")
    End Try

    If StartCounter > EndCounter Then
        MsgBox("Invalid Values")
        Exit Sub
    End If

    Consumption = EndCounter - StartCounter

    Dim strConnection As String = _
     		"Data Source=(local);" & _
     		"Database='UtilityCompany1';" & _
     		"Integrated Security=yes;"
    Dim ExecuteFunction As String = "DECLARE @Difference int " & _
      		"SET @Difference = " & Consumption.ToString() & _
      		"SELECT dbo.EvaluateInvoice(@Difference);"

    Using connection As SqlConnection = New SqlConnection(strConnection)
        Dim command As SqlCommand = _
             New SqlCommand(ExecuteFunction, connection)

        connection.Open()
        Dim rdr As SqlDataReader = command.ExecuteReader()

        txtConsumption.Text = Consumption.ToString()

        While rdr.Read()
            txtAmountDue.Text = rdr(0)
         End While
        rdr.Close()
    End Using
End Sub

Execute the application
Click the Database button to create the function and click OK
Enter some values for the start and end counter
Click Evaluate. Here is an example:
Close the form and return to your programming environment

Change the code of the Database button as follows:

Private Sub btnDatabase_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles btnDatabase.Click
    Dim strConnection As String = _
        "Data Source=(local);Integrated Security=yes"

    Using connection As SqlConnection = New SqlConnection(strConnection)
        Dim command As SqlCommand = _
	        New SqlCommand("DROP DATABASE UtilityCompany1;", _
        		       connection)

        connection.Open()
        Command.ExecuteNonQuery()
        MsgBox("The UtilityCompany1 database has been deleted.")
    End Using
End Sub

Execute the application
Click the Database button
Close the form and return to your programming environment

Introduction to Built-Functions

Overview

While your primary job as a database developer consists of creating lists, probably your second most important job is to assist your users with the various assignments they must perform on your application. One way you can assist is to use functions that perform otherwise complex tasks. To assist your development with the different tasks of a database, Transact-SQL ships with various already created and tested functions. You just need to be aware of these functions, their syntax, and the results they produce.

Because of their complexities, some values can be easily recognized or fixed. For example, a date such as January 6, 1995 is constant and can never change. This type of value is referred to as deterministic because it is always the same. In the same way, a time value such as 5 PM is constant and cannot change. There are other values that cannot be known in advance because they change based on some circumstances. For example, the starting date of the school year changes from one year to another but it always occurs. This means that, you know it will happen but you do not know the exact date. Such a value is referred to as non-deterministic.

To support determinism and non-determinism, Transact-SQL provides two broad categories of functions. A function that always returns the same or known value is referred to as deterministic. A function whose returned value may depend on a condition is referred to as non-deterministic.

Practical Learning: Introducing Built-in Functions

Create a new Windows Application named Payroll
From the Common Controls section of the Toolbox, add a Button to the form

Double-click the button and implement its Click event as follows:

Imports System.Data.SqlClient

Public Class Exercise

    Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) _
                              Handles Button1.Click
        Dim strConnection As String = _
 		"Data Source=(local);Integrated Security=yes"

        Using connection As SqlConnection = New SqlConnection(strConnection)
            Dim command As SqlCommand = _
                New SqlCommand("CREATE DATABASE Payroll;", connection)

            connection.Open()
            command.ExecuteNonQuery()

            MsgBox("A database named ""Payroll"" has been created.")
        End Using
    End Sub
End Class

Execute the application
Click the button
Click OK
Close the form and return to your programming environment
Delete the button

Casting a Value

In most cases, a value the user submits to your database is primarily considered a string. This is convenient if that's what you are expecting. If the value the user provides must be treated as something other than a string, for example, if the user provides a number, before using such a value, you should first convert it to the appropriate type, that is, from a string to the expected type.

To assist with conversion, you can use either the CAST() or the CONVERT() function. The syntax of the CAST() function is:

CAST(Expression AS DataType)

The Expression is the value that needs to be cast. The DataType factor is the type of value you want to convert the Expression to. The DataType can be one of those we reviewed in Lesson 20.

In the following example, two variables are declared and initialzed as strings. Because they must be involved in a multiplication, each is converted to a Decimal type.

Practical Learning: Casting a Value

Design the form as follows:

Control	Text	Name	Other Properties
Label	Hourly Salary:
TextBox	0.00	txtHourlySalary	TextAlign: Right
Label	Weekly Hours:
TextBox	0.00	txtWeeklyHours	TextAlign: Right
Button	Calculate	btnCalculate
Label	Weekly Salary:
TextBox	0.00	txtWeeklySalary	TextAlign: Right
Button	Close	btnClose

Double-click the Calculate button and implement the Click event as follows:

Imports System.Data.SqlClient

Public Class Exercise

    Private Sub btnCalculate_Click(ByVal sender As System.Object, _
                                   ByVal e As System.EventArgs) _
                                   Handles btnCalculate.Click
        Dim HourlySalary As Double
        Dim WeeklyHours As Double

        Try
            HourlySalary = CDbl(txtHourlySalary.Text)
        Catch
            MsgBox("Invalid Weekly Hours")
        End Try

        Try
            WeeklyHours = CDbl(txtWeeklyHours.Text)
        Catch
            MsgBox("Invalid Weekly Hours")
        End Try

        Dim strConnection As String = _
     		"Data Source=(local);" & _
     		"Database='Payroll';" & _
     		"Integrated Security=yes;"
        Dim ExecuteFunction As String = _
         	"DECLARE @StrSalary Varchar(10), " & _
         	"@StrHours Varchar(6), " & _
         	"@WeeklySalary Decimal(6,2) " & _
         	"SET @StrSalary = '" & HourlySalary.ToString() & "'; " & _
         	"SET @StrHours = '" & WeeklyHours.ToString() & "'; " & _
         "SET @WeeklySalary = CAST(@StrSalary As Decimal(6,2)) * " & _
         	"CAST(@StrHours As Decimal(6,2)); " & _
         	"SELECT @WeeklySalary;"

        Using connection As SqlConnection = New SqlConnection(strConnection)
            Dim command As SqlCommand = _
   		New SqlCommand(ExecuteFunction, connection)

            connection.Open()
            Dim rdr As SqlDataReader = Command.ExecuteReader()

            While rdr.Read()
                txtWeeklySalary.Text = rdr(0)
            End While

            rdr.Close()
        End Using
    End Sub
End Class

Execute the application
Enter a decimal value for the side and click the Calculate button. Here is an example:
Close the form and return to your programming environment
In the Class Name combo box, select btnClose

In the Method Name combo box, select Click and implement the event as follows:

Private Sub btnClose_Click(ByVal sender As Object, _
                               ByVal e As System.EventArgs) _
                               Handles btnClose.Click
    Dim strConnection As String = _
	   "Data Source=(local);Integrated Security=yes"

    Using connection As SqlConnection = New SqlConnection(strConnection)
        Dim command As SqlCommand = _
                New SqlCommand("DROP DATABASE Payroll;", connection)

        connection.Open()
        command.ExecuteNonQuery()
        MsgBox("A database named ""Payroll"" has been deleted.")
    End Using
End Sub

Execute the application
Click the Close button
Close the form and return to your programming environment

Change the code of the Close button as follows:

Private Sub btnClose_Click(ByVal sender As Object, _
                               ByVal e As System.EventArgs) _
                               Handles btnClose.Click
    End
End Sub

Save all

Converting a Value

Like CAST(), the CONVERT() function is used to convert a value. Unlike CAST(), CONVERT can be used to convert a value its original type into a non-similar type. For example, you can use CONVERT to cast a number into a string and vice-versa.

The syntax of the CONVERT() function is:

CONVERT(DataType [ ( length ) ] , Expression [ , style ])

The first argument must be a known data type, such as those we reviewed in Lesson 4. If you are converting the value into a string (varchar, nvarchar, char, nchar) or a binary type, you should specify the number of allowed characters the data type's own parentheses. As reviewed for the CAST() function, the Expression is the value that needs to be converted.

Practical Learning: Converting a Value

Start a new Windows Application named Square1

Design the form as follows:

Control	Text	Name	Other Properties
Label	Side:
TextBox	0.00	txtSide	TextAlign: Right
Button	Calculate	btnCalculate
Label	Perimeter:
TextBox	0.00	txtPerimeter	TextAlign: Right
Button	Close	btnClose
Label	Area:
TextBox	0.00	txtArea	TextAlign: Right

Right-click the form and click View Code and implement its Load event as follows:

Imports System.Data.SqlClient

Public Class Execute
    Private Sub Execute_Load(ByVal sender As Object, _
                             ByVal e As System.EventArgs) _
                             Handles Me.Load
        Dim strConnection As String = _
    		"Data Source=(local);Integrated Security=yes"

        Using connection As SqlConnection = New SqlConnection(strConnection)
            Dim command As SqlCommand = _
                    New SqlCommand("CREATE DATABASE Geometry1;", connection)

            connection.Open()
            command.ExecuteNonQuery()
            MsgBox("A database named ""Geometry1"" has been created.")
        End Using
    End Sub
End Class

Execute the application to create the database
Close the form and return to your programming environment
Delete all the code inside the Load event
In the Class Name combo box, select btnCalculate

In the Method Name combo box, select Click implement the event as follows:

Private Sub btnCalculate_Click(ByVal sender As Object, _
                               ByVal e As System.EventArgs) _
                               Handles btnCalculate.Click
    Dim dSide As Double

    Try
        dSide = CDbl(txtSide.Text)
    Catch
        MsgBox("Invalid Side Value")
    End Try

    Dim strConnection As String = _
    	  "Data Source=(local);" & _
   	  "Database='Geometry1';" & _
     	  "Integrated Security=yes;"
    Dim ExecuteFunction As String = _
    	    	"DECLARE @Side As Decimal(10,3), " & _
        	"        @Perimeter As Decimal(10,3), " & _
        	"        @Area As Decimal(10,3); " & _
        	"SET     @Side = " & dSide.ToString() & "; " & _
        	"SET     @Perimeter = @Side * 4; " & _
        	"SET     @Area = @Side * @Side; " & _
        	"SELECT CONVERT(varchar(10), @Perimeter, 10), " & _
        "       CONVERT(varchar(10), @Area, 10);"

    Using connection As SqlConnection = _
    		New SqlConnection(strConnection)
        Dim command As SqlCommand = _
             New SqlCommand(ExecuteFunction, connection)

        connection.Open()
        Dim rdr As SqlDataReader = command.ExecuteReader()

        While rdr.Read()
            txtPerimeter.Text = rdr(0)
            txtArea.Text = rdr(1)
        End While
        rdr.Close()
    End Using
End Sub

Execute the application
Enter a decimal value for the side and click the Calculate button. Here is an example:
Close the form and return to your programming environment
In the Class Name combo box, select btnClose

In the Method Name combo box, select Click implement the event as follows:

Private Sub btnClose_Click(ByVal sender As Object, _
                           ByVal e As System.EventArgs) _
                           Handles btnClose.Click
    Dim strConnection As String = _
	     "Data Source=(local);Integrated Security=yes"

    Using connection As SqlConnection = New SqlConnection(strConnection)
        Dim command As SqlCommand = _
                New SqlCommand("DROP DATABASE Geometry1;", connection)

        connection.Open()
        command.ExecuteNonQuery()
        MsgBox("The ""Geometry1"" database named has been deleted.")
    End Using
End Sub

Execute the application and click the Close button to close it
Close the form and return to your programming environment

Change the code of the Close button as follows:

Private Sub btnClose_Click(ByVal sender As Object, _
                           ByVal e As System.EventArgs) _
                           Handles btnClose.Click
        End
End Sub

Save all

Lesson Summary

Exercises

Write a function named ProcessPayroll1 that takes the number of hours worked in a week. Then the function returns a value that represents overtime. If the employee worked less than 40 hours, there is no overtime. If the employee worked for more than 40 hours, the number over 40 is considered overtime
Write a function named GetWeekdayName that, when given a date, can find and display the name of the week for that date
Write a function named GetNumberDays that takes two dates and returns the number of days between them
Write a function named AddNumberDays that takes a date and an integer, then it returns the date added that number


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