Microsoft ADO .NET 4 Step by Step - p 12 potx

This prevents parent records Flights from being deleted or having their ID col-umn values changed if related child rows Legs exist.. Not only did the first row in the Flights field have

86 Microsoft ADO NET 4 Step by Step

6 Test setting linked child fields to NULL when a parent record is deleted Set the Delete Rule field to SetNull Select the first row in the Flights field, the one with ID 759 Click

the (Flights) Delete button and confirm the delete action

7 Review the Flights and Legs fields Although the first row in the Flights field has been

removed, its child records in the Legs field remain However, their FlightID column

val-ues have been cleared and are set to NULL Those records no longer have a parent row

Defining the Update and Delete Rules in a DataRelation: Visual Basic

Note This exercise uses the “Chapter 5 VB” sample project and continues the previous exercise

in this chapter.

1 Open the source code view for the FlightInfo form Locate the RefreshConstraints

method The application lets the user alter the rules for adjusting the child table when

changes are made to the parent table The RefreshConstraints routine updates the

relevant constraint with the user’s rule choice

2 Just after the “Alter its cascade rules” comment, add the following statements:

linkConstraint.DeleteRule = CType(DeleteRule.SelectedItem, Data.Rule)

linkConstraint.UpdateRule = CType(UpdateRule.SelectedItem, Data.Rule)

3 Run the program The Update Rule and Delete Rule fields are both set to None by

default This prevents parent records (Flights) from being deleted or having their ID

col-umn values changed if related child rows (Legs) exist

4 Test cascade updates Set the Update Rule field to Cascade Select the first row in the

Flights field, the one with ID 834 Click the (Flights) Edit button Use the Edit Flight form that appears to alter the Flight ID value from 834 to another value, such as 759 Click

OK on that editor form

5 Review the Flights and Legs fields Not only did the first row in the Flights field have its

ID value changed to 759 but the FlightID values for the related rows in the Legs field

changed to 759 as well.

6 Test setting linked child fields to NULL when a parent record is deleted Set the Delete Rule field to SetNull Select the first row in the Flights field, the one with ID 759 Click

the (Flights) Delete button and confirm the delete action

7 Review the Flights and Legs fields Although the first row in the Flights field has been

removed, its child records in the Legs field remain However, their FlightID column

val-ues have been cleared and are set to NULL Those records no longer have a parent row

Chapter 5 Bringing Related Data Together 87

Summary

This chapter demonstrated how individual DataTable instances can be joined together in an ADO.NET DataSet Each data table object includes many features that let you query and

manipulate the data in its rows By bringing distinct tables together in a data set, you gain additional features that affect multiple tables simultaneously and, if desired, automatically

The DataRelation class defines the link between columns in two different tables This class defines only the relationship; it doesn’t enforce the rules of the relationship Constraint ob-jects, specifically the UniqueConstraint and ForeignKeyConstraint derived classes, impose the

data requirements needed to ensure data integrity and data expectations between linked tables

88 Microsoft ADO NET 4 Step by Step

Chapter 5 Quick Reference

Add a table to a DataSet Define a DataSet instance.

Define a DataTable instance, adding columns and rows as needed Call the DataSet object’s Tables.Add method, passing it the instance

of the DataTable.

Link two DataTable objects in a

relation-ship

Define a DataSet instance.

Define two DataTable instances, adding columns and rows as

needed.

Determine which columns from each table will form the relationship link.

Add both tables to the DataSet using the Tables.Add method Create a DataRelation instance, passing instances of the columns to

be linked to its constructor.

Call the DataSet object’s Relations.Add method, passing it the in-stance of the DataRelation.

Enforce cascade deletes in a

parent-child relationship

Locate the DataRelation instance that defines the link relationship Set the DataRelation object’s DeleteRule to System.Data.Rule Cascade.

Locate the parent row for a child row Ensure that the tables are linked with a DataRelation.

Call the child DataRow object’s GetParentRow method, passing it the name of the DataRelation that defines the link relationship.

Locate the child rows for a parent row Ensure that the tables are linked with a DataRelation.

Call the parent DataRow object’s GetChildRow method, passing it the name of the DataRelation that defines the link relationship.

89

Chapter 6

Turning Data into Information

After completing this chapter, you will be able to:

■

■ Return a value that aggregates data from a table column

■

■ Add a column that aggregates data from a table, or from its parent or child table

■

■ Build an index-based view of a table

■

■ Generate a new table based on a projected view of the original table

After you have joined DataTable instances together in a DataSet, ADO.NET enables a few

more features that let you use those table relationships to analyze and select data These features build upon some of the single-table functions covered in earlier chapters

This chapter introduces the data-aggregation features included in the ADO.NET Framework, expressions that summarize data across multiple table rows Although not as powerful as

the aggregation features found in relational database systems, the DataTable variations still

provide quick access to multirow data summaries The chapter ends with an introduction to

the DataView class, which lets you establish row selection, filtering, and sorting standards for

a DataTable.

Note The exercises in this chapter all use the same sample project, a tool that demonstrates aggregate and data view features Although you will be able to run the application after each exercise, the expected results for the full application might not appear until you complete all exercises in the chapter.

Aggregating Data

An aggregation function returns a single calculated value from a set of related values

Averages are one type of data aggregation; they calculate a single averaged value from an input of multiple source values ADO.NET includes seven aggregation functions for use in

expression columns and other DataTable features.

■

■ Sum Calculates the total of a set of column values The column being summed must

be numeric, either integral or decimal

■

■ Avg Returns the average for a set of numbers in a column This function also requires

a numeric column

■

■ Min Indicates the minimum value found within a set of column values Numbers, strings, dates, and other types of data that can be placed in order are all valid for the target column

■

■ Max Like Min, but returns the largest value from the available column values As with the Min function, most column types will work.

■

■ Count Simply counts the number of rows included in the aggregation You can pass any type of column to this function As long as a row includes a non-NULL value in that column, it will be counted as 1

■

■ StDev Determines the statistical standard deviation for a set of values, a common measure of variability within such a set The indicated column must be numeric

■

■ Var Calculates the statistical variance for a set of numbers, another measurement re-lated to the standard deviation Only numeric columns are supported

These seven data aggregation features appear as functions within ADO.NET expressions Expressions were introduced in the “Using Expression Columns” section of Chapter 4,

“Accessing the Right Data Values.” String expressions form the basis of custom expression

col-umns and are also used in selecting subsets of DataTable rows To aggregate data, use one of

the following function formats as the expression string:

■

■ Sum(column-name)

■

■ Avg(column-name)

■

■ Min(column-name)

■

■ Max(column-name)

■

■ Count(column-name)

■

■ StDev(column-name)

■

■ Var(column-name)

In ADO.NET, aggregates always summarize a single DataTable column Each aggregate

func-tion considers only non-NULL column values Rows that contain NULL values in the specified column are excluded from the aggregation For example, if you take the average of a table column with 10 rows, but 3 of those rows contain NULL values in the column being averaged,

the function will average only the 7 non-NULL values This is especially useful with the Count

function; it counts only the number of rows that have a non-NULL value for the passed column name If all the column values are NULL, or if there are no rows to apply to the aggregation

function, the result is NULL (System.DBNull).

Chapter 6 Turning Data into Information 91

Generating a Single Aggregate

To calculate the aggregate of a single table column, use the DataTable object’s Compute

method Pass it an expression string that contains an aggregate function with a column-name argument

C#

DataTable employees = new DataTable("Employee");

employees.Columns.Add("ID", typeof(int));

employees.Columns.Add("Gender", typeof(string));

employees.Columns.Add("FullName", typeof(string));

employees.Columns.Add("Salary", typeof(decimal));

// - Add employee data to table, then

decimal averageSalary = (decimal)employees.Compute("Avg(Salary)", "");

Visual Basic

Dim employees As New DataTable("Employee")

employees.Columns.Add("ID", GetType(Integer))

employees.Columns.Add("Gender", GetType(string))

employees.Columns.Add("FullName", GetType(String))

employees.Columns.Add("Salary", GetType(Decimal))

' - Add employee data to table, then

Dim averageSalary As Decimal = CDec(employees.Compute("Avg(Salary)", ""))

In the preceding code, the Compute method calculates the average of the values in the Salary column The second argument to Compute is a filter that limits the rows included in the calculation It accepts a Boolean criteria expression similar to those used in the DataTable Select method call.

C#

int femalesInCompany = (int)employees.Compute("Count(ID)",

"Gender = 'F'");

Visual Basic

Dim femalesInCompany As Integer = CInt(employees.Compute("Count(ID)",

"Gender = 'F'"))

92 Microsoft ADO NET 4 Step by Step

Computing an Aggregate Value: C#

1 Open the “Chapter 6 CSharp” project from the installed samples folder The project

in-cludes three Windows.Forms classes: Switchboard, Aggregates, and DataViews.

2 Open the source code view for the Aggregates form Locate the ActCompute_Click

func-tion This routine computes an aggregate value for a single table column

3 Just after the “Build the expression” comment, add the following statement:

expression = ComputeFunction.SelectedItem.ToString() + "(" +

columnName + ")";

This code builds an expression string that combines one of the seven aggregate func-tions and a column name from the sample table

4 Just after the “Process the expression” comment, add the following code:

try

{

result = whichTable.Compute(expression, "");

}

catch (Exception ex)

{

MessageBox.Show("Could not compute the column: " + ex.Message);

return;

}

The code performs the calculation in a try block because the code that built the

ex-pression didn’t bother to verify things such as allowing only numeric columns to be

used with the Sum aggregate function The catch block will capture such problems at

runtime

5 Just after the “Display the results” comment, add the following statements:

if (DBNull.Value.Equals(result))

MessageBox.Show("NULL");

else

MessageBox.Show(result.ToString());

Some aggregates may return a NULL result depending on the contents of the column This code makes that distinction

6 Run the program When the Switchboard form appears, click Aggregate Functions

When the Aggregates form appears, use the fields to the right of the Compute label to

generate the aggregate For example, select Sum from the Aggregate Function field (the one just to the right of the Compute label), and choose Child.Population2009

from the Column Name field (the one in parentheses) Then click Compute The response of “307006550” comes from adding up all values in the child table’s

Population2009 column.

Chapter 6 Turning Data into Information 93

Note The “Child.” prefix shown in the Column Name field is stripped out before the column

name is inserted into the expression The Compute method does not support the Parent and Child prefixes before column names.

Computing an Aggregate Value: Visual Basic

1 Open the “Chapter 6 VB” project from the installed samples folder The project includes

three Windows.Forms classes: Switchboard, Aggregates, and DataViews.

2 Open the source code view for the Aggregates form Locate the ActCompute_Click

func-tion This routine computes an aggregate value for a single table column

3 Just after the “Build the expression” comment, add the following statement:

expression = ComputeFunction.SelectedItem.ToString() & "(" &

columnName & ")"

This code builds an expression string that combines one of the seven aggregate func-tions and a column name from the sample table

4 Just after the “Process the expression” comment, add the following code:

Try

result = whichTable.Compute(expression, "")

Catch ex As Exception

MessageBox.Show("Could not compute the column: " & ex.Message)

Return

End Try

94 Microsoft ADO NET 4 Step by Step

The code performs the calculation in a Try block because the code that built the

ex-pression didn’t bother to verify things such as allowing only numeric columns to be

used with the Sum aggregate function The Catch block will capture such problems at

runtime

5 Just after the “Display the results” comment, add the following statements:

If (IsDBNull(result) = True) Then

MessageBox.Show("NULL")

Else

MessageBox.Show(result.ToString())

End If

Some aggregates may return a NULL result depending on the contents of the column This code makes that distinction

6 Run the program When the Switchboard form appears, click Aggregate Functions

When the Aggregates form appears, use the fields to the right of the Compute label to

generate the aggregate For example, select Sum from the Aggregate Function field (the one just to the right of the Compute label), and choose Child.Population2009

from the Column Name field (the one in parentheses) Then click Compute The response of “307006550” comes from adding up all values in the child table’s

Population2009 column.

Note In the example, the “Child.” prefix shown in the Column Name field is stripped out before

the column name is inserted into the expression The Compute method does not support the Parent and Child prefixes before column names.

Adding an Aggregate Column

Expression columns typically compute a value based on other columns in the same row You can also add an expression column to a table that generates an aggregate value In the absence of a filtering expression, aggregates always compute their totals using all rows in a table This is also true of aggregate expression columns When you add such a column to a table, that column will contain the same value in every row, and that value will reflect the ag-gregation of all rows in the table

Chapter 6 Turning Data into Information 95 C#

DataTable sports = new DataTable("Sports");

sports.Columns.Add("SportName", typeof(string));

sports.Columns.Add("TeamPlayers", typeof(decimal));

sports.Columns.Add("AveragePlayers", typeof(decimal),

"Avg(TeamPlayers)");

sports.Rows.Add(new Object[] {"Baseball", 9});

sports.Rows.Add(new Object[] {"Basketball", 5});

sports.Rows.Add(new Object[] {"Cricket", 11});

MessageBox.Show((string)sports.Rows[0]["AveragePlayers"]); // Displays 8.3

MessageBox.Show((string)sports.Rows[1]["AveragePlayers"]); // Also 8.3

Visual Basic

Dim sports As New DataTable("Sports")

sports.Columns.Add("SportName", GetType(String))

sports.Columns.Add("TeamPlayers", GetType(Decimal))

sports.Columns.Add("AveragePlayers", GetType(Decimal),

"Avg(TeamPlayers)")

sports.Rows.Add({"Baseball", 9})

sports.Rows.Add({"Basketball", 5})

sports.Rows.Add({"Cricket", 11})

MessageBox.Show(CStr(sports.Rows(0)!AveragePlayers)) ' Displays 8.3

MessageBox.Show(CStr(sports.Rows(1)!AveragePlayers)) ' Also 8.3

Aggregating Data Across Related Tables

Adding aggregate functions to an expression column certainly gives you more data options,

but as a calculation method it doesn’t provide any benefit beyond the DataTable.Compute

method The real power of aggregate expression columns appears when working with

relat-ed tables By adding an aggregate function to a parent table that references the child table, you can generate summaries that are grouped by each parent row This functionality is

simi-lar in purpose to the GROUP BY clause found in the SQL language.

To apply an aggregate to a table relationship, you first add both tables to a DataSet and then add the relevant DataRelation between the linked fields After the tables are linked, you in-clude the Child keyword with the aggregate function’s column name reference.

function-name(Child.column-name)