Accessing the Right Data Values

Chapter 4Accessing the Right Data Values After completing this chapter, you will be able to: ■ ■ Find items in a DataTable by primary key ■ ■ Search for DataRow instances using standard

Chapter 4

Accessing the Right Data Values

After completing this chapter, you will be able to:

■

■ Find items in a DataTable by primary key

■

■ Search for DataRow instances using standard query statements

■

■ Obtain a set of DataRow objects sorted by one or more columns

■

■ Add expression columns to a DataTable that present calculated values

Although adding records to a DataTable is important, the real value of ADO.NET lies in

get-ting those records back out in a variety of ways Fortunately, the data framework includes many different methods and tools to fulfill that very purpose

This chapter introduces a few of the most basic tools, all of which appear in the DataTable,

DataColumn, and DataRow classes you’ve already met Each data table includes features that

let you select just the records you need These features are flexible enough to rival those you might find in traditional databases The chapter also includes a discussion of “expression col-umns,” a way to add useful values to each table row without adding any actual data

Note The exercises in this chapter all use the same sample project, a tool that queries data

from a sample DataTable 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.

Both forms in the sample application use the DataGridView control, one of the standard controls

provided with Visual Studio Chapter 21, “Binding Data with ADO.NET,” discusses the ADO.NET-specific features of this control.

Querying and Sorting Data

In Chapter 3, “Storing Data in Memory,” you learn how to iterate through all the records in a

DataTable object’s Rows collection However, there are times when you need to access only

specific rows, often based on applying search criteria to one or more of the table’s columns Although you can scan through every row in the table, checking each record as you

encoun-ter it to see whether it matches your search limits, the DataTable class already includes fea-tures that will let you select just those DataRow instances that match a selection rule

Each DataTable can include an optional primary key definition, a collection of DataColumn objects assigned to the table’s PrimaryKey member This key is often a unique value from a

single column, but tables also support multicolumn keys After you define a table’s primary key, all rows added to that table must have a non-NULL, unique key

To locate a row based on its primary key, use the table’s Rows.Find method For tables with

single-column keys, pass this method a key value of the appropriate data type For

multi-column keys, pass the key components as an array Find returns a single DataRow instance for

the matching row

C#

// - Single-part key

DataRow matchingRow = someTable.Rows.Find(searchValue);

// - Multi-part key

DataRow matchingRow = someTable.Rows.Find(new Object[]

{keyPart1, keyPart2, keyPart3});

Visual Basic

' - Single-part key

Dim matchingRow As DataRow = someTable.Rows.Find(searchValue)

' - Multi-part key

Dim matchingRow As DataRow = someTable.Rows.Find({keyPart1,

keyPart2, keyPart3})

If no row matches the provided primary key, Find returns Nothing (in Visual Basic) or null (in

C#) The method throws an exception if you apply it to tables with no defined primary key

Note It is possible to add two rows with the same primary key to a DataTable by disabling its constraints (as discussed in Chapter 5, “Bringing Related Data Together”) In such tables, the Find

method returns only the first row with a matching primary key value.

Finding a Row by Primary Key: C#

1 Open the “Chapter 4 CSharp” project from the installed samples folder The project

in-cludes two Windows.Forms classes: TableExaminer and ResultsViewer.

2 Open the source code view for the TableExaminer form Locate the ActPrimaryKey_Click

event handler This routine obtains a long-integer value from the user and then uses it

as the primary key lookup value in the application’s sample DataTable Most of the code

exists to ensure that the user provides a valid ID

3 Locate the try catch statement just after the “Perform the lookup” comment In the try

block, add the following statement:

result = workTable.Rows.Find(usePrimaryKey);

This line performs the actual primary-key lookup, returning the DataRow of the match-ing record, or null when the key doesn’t match any of the table’s primary keys.

4 Run the program On the Lookup By Primary Key tab, enter a value in the Primary Key

field (try 2352), and then click the Lookup button The matching row appears in a

sepa-rate window (not shown here)

Finding a Row by Primary Key: Visual Basic

1 Open the “Chapter 4 VB” project from the installed samples folder The project includes

two Windows.Forms classes: TableExaminer and ResultsViewer.

2 Open the source code view for the TableExaminer form Locate the ActPrimaryKey_Click

event handler This routine obtains a long-integer value from the user and then uses it

as the primary key lookup value in the application’s sample DataTable Most of the code

exists to ensure that the user provides a valid ID

3 Locate the Try Catch statement just after the “Perform the lookup” comment In the

Try block, add the following statement:

result = workTable.Rows.Find(usePrimaryKey)

This line performs the actual primary-key lookup, returning the DataRow of the match-ing record, or Nothmatch-ing when the key doesn’t match any of the table’s primary keys.

4 Run the program On the Lookup By Primary Key tab, enter a value in the Primary Key

field (try 2352), and then click Lookup The matching row appears in a separate window,

as shown here

Selecting Rows with a Search Criteria

The Find method is useful when you need to retrieve a single row based on a primary key

lookup value, but useful data analysis typically involves searching across many of a table’s

columns and returning all possible matches To provide this functionality, the DataTable class includes the Select method.

Note When you anticipate issuing the same Select request on a table multiple times, it’s more efficient to create a DataView that provides a limited presentation of the table’s rows Chapter 6,

“Turning Data into Information,” introduces the DataView class and its use in presenting content

from a data table.

You pass the Select method a string that contains the selection criteria When successful, the method returns an array of matching DataRow instances from the table.

C#

DataRow[] matchingRows = someTable.Select(filterCriteria);

Visual Basic

Dim matchingRows() As DataRow = someTable.Select(filterCriteria)

Note You can iterate through the returned array as your processing needs require Although the rows come to you packaged in an array, they are still part of the original table Any changes you make to these rows affect the underlying table.

The filter expression passed to the Select method uses a SQL-like syntax to build a Boolean

statement that will either match or not match specific rows in the table Any of the columns

in your DataTable object is fair game for comparisons As an example, the following expres-sion will return all rows with a Salary column value of at least 100,000:

Salary >= 100000

Columns can be compared to each other and standard mathematical expressions can en-hance the column elements

Bonus > Salary * 0.15

You can string together multiple criteria using the Boolean operators AND, OR, and NOT, and

use parentheses to force evaluation in a specific order

Age >= 18 AND (InSchool = True OR LivingAtHome = True)

Table 4-1 lists the some of the elements you can use in filter expressions To view the full doc-umentation for filter expressions, access the Visual Studio online help entry for “DataColumn Expression Property.”

TABLE 4-1 Filter Expression Elements

Column names Any of the column names from the DataTable Surround column

names that contain embedded space characters or other

non-alphanumeric characters with a set of square brackets, as in [Full Name] for a column named Full Name.

<, >, <=, >=, <>, = Use the standard comparison operators to compare columns to

literal values, to each other, or to more complex expressions.

IN Match from a collection of comma-delimited elements.

BillDenomination IN (5, 10, 20) LIKE Match a string pattern The pattern can include zero or more

oc-currences of the wildcard character (“*” or “%”), but at the ends of the pattern string only, not in the middle.

ProductClass = 'AA*' or:

ProductClass = 'AA%' AND, OR, NOT Use these Boolean operators to join multiple expressions together Parentheses Force the order of expression evaluation with parentheses.

Event Name Triggering Action

Literals Literals include integers, decimals, numbers in scientific notation,

strings in single quotes, and dates or times in # marks.

CONVERT Convert an expression or column from one data type to another.

CONVERT(expression, new-type)

The list of allowed data types is pretty close to those allowed when creating data columns There are also restrictions on which data types can be coerced into other types See the Visual Studio online help for full details.

LEN Returns the length of a string column or expression.

ISNULL Returns an expression or a default expression if the first argument

evaluates to NULL Useful for ensuring that a NULL value does not appear in a calculation For example, the following

expres-sion compares the FamilyMembers column to the value 2 when FamilyMembers is not NULL However, if FamilyMembers evaluates

to NULL, it defaults to 1 instead.

ISNULL(FamilyMembers, 1) >= 2 IIF The ternary conditional function, similar to the If and IIf operators

in Visual Basic, and to the :? operator in C# The operator contains

three arguments If the first argument evaluates to true, the

func-tion returns the second argument, the “true” part Otherwise, it returns the third argument, the “false” part.

IIF(Age >= 18, 'Adult', 'Minor') TRIM Trims whitespace from the ends of a string column or expression SUBSTRING Returns a portion of a string column or expression, starting from a

1-based position and continuing on for a specific length count SUBSTRING(PhoneNumber, 1, 3)

Sorting Search Results

By default, the DataTable.Select method returns DataRow objects in the order in which they

were added to the table To sort the results based on one or more columns in the returned

rows, send a second string argument to the Select method that indicates the sort rules.

C#

DataRow[] sortedRows = someTable.Select(filterCriteria, sortRules);

Visual Basic

Dim sortedRows() As DataRow = someTable.Select(filterCriteria, sortRules)

The sort string contains a comma-delimited list of the columns to be used for sorting, from

left to right Each column can be optionally followed by ASC for an ascending sort on that column or DESC for a descending sort; ascending is the default The following sort expression

orders the returned rows by descending OrderDate and then by (ascending) customer name:

OrderDate DESC, CustomerName

A third argument to the Select method lets you limit the results based on the state

of each row In tables that have had row-level changes, but for which you haven’t yet

called AcceptResults, this feature can return just the deleted rows, or just the unchanged

rows, among other options See the Visual Studio online help entry “DataViewRowState Enumeration” for a complete list of available options

Selecting and Sorting DataRow Objects: C#

Note This exercise uses the “Chapter 4 CSharp” sample project and continues the previous exer-cise in this chapter.

1 Open the source code view for the TableExaminer form Locate the ActCriteria_Click

event handler This routine collects user-supplied selection and sorting expressions;

then uses them to obtain a set of DataRow instances from a DataTable Most of the

code exists to ensure that the user provides valid expressions

2 Locate the try catch statement just after the “Apply the filter and sorting list” comment

In the try block, add the following statement:

results = workTable.Select(CriteriaFilter.Text, CriteriaSorting.Text);

This line performs the actual row selection, returning the optionally sorted DataRow

instances, or an empty array when the selection expression doesn’t match any of the table’s rows

3 Run the program On the Lookup By Criteria tab, provide expressions that will return

a list of students with improving grades, sorted by name Enter ScoreTrimester3 >

ScoreTrimester1 OR ScoreTrimester3 > ScoreTrimester2 in the Filter Criteria field,

and StudentName in the Sorting List field Click Lookup The matching rows appear in

a separate window

Selecting and Sorting DataRow Objects: Visual Basic

Note This exercise uses the “Chapter 4 VB” sample project and continues the previous exercise

in this chapter.

1 Open the source code view for the TableExaminer form Locate the ActCriteria_Click

event handler This routine collects user-supplied selection and sorting expressions;

then uses them to obtain a set of DataRow instances from a DataTable Most of the

code exists to ensure that the user provides valid expressions

2 Locate the Try Catch statement just after the “Apply the filter and sorting list”

com-ment In the Try block, add the following statement:

results = workTable.Select(CriteriaFilter.Text, CriteriaSorting.Text)

This line performs the actual row selection, returning the optionally sorted DataRow

instances or an empty array when the selection expression doesn’t match any of the table’s rows

3 Run the program On the Lookup By Criteria tab, provide expressions that will return

a list of students with improving grades, sorted by name Enter ScoreTrimester3 >

ScoreTrimester1 OR ScoreTrimester3 > ScoreTrimester2 in the Filter Criteria field,

and StudentName in the Sorting List field Click Lookup The matching rows appear in

a separate window

Performing Case-Sensitive Lookups

The Select method ignores character casing by default when comparing string values For

instance, the following expression will match joe, Joe, JOE, or any other mixed-case variation

on the name:

FirstName = 'joe'

To enforce case-sensitive matches on all searches instead, set the table’s CaseSensitive

property

C#

someTable.CaseSensitive = true;

Visual Basic

someTable.CaseSensitive = True

Using Expression Columns

In Chapter 2, “Building Tables of Data,” you learned how to add columns to a table that

would each hold data values of a specific type These static columns define the core data within a table The DataTable class also supports expression columns, fields that expose a

cal-culated result based on the data in other row columns For instance, if your table of orders

includes a Subtotal column and a Tax column, you could add an expression column named

Total that calculated the sum of Subtotal and Tax.

To add an expression column to a table, create a standard DataColumn object, fill in its

ColumnName and DataType properties, and then assign a string expression that performs the

custom calculation to the Expression property.

C#

// - Syntax using a DataColumn object

DataColumn orderTotal = new DataColumn();

orderTotal.ColumnName = "Total";

orderTotal.DataType = typeof(decimal);

orderTotal.Expression = "Subtotal + ISNULL(Tax, 0)";

someTable.Columns.Add(orderTotal);

// - Syntax using Add arguments only

someTable.Columns.Add("Total", typeof(decimal),

"Subtotal + ISNULL(Tax, 0)");

Visual Basic

' - Syntax using a DataColumn object

Dim orderTotal As New DataColumn

orderTotal.ColumnName = "Total"

orderTotal.DataType = GetType(Decimal)

orderTotal.Expression = "Subtotal + ISNULL(Tax, 0)"

someTable.Columns.Add(orderTotal)

' - Syntax using Add arguments only

someTable.Columns.Add("Total", GetType(Decimal),

"Subtotal + ISNULL(Tax, 0)")

The expression field uses the same elements from Table 4-1 that you used with the

DataTable.Select method To view the full documentation for this expression, access the

Visual Studio online help entry for “DataColumn.Expression Property.”

Note The documentation for the Expression property discusses “aggregate functions.” These are

covered in Chapter 6.

After being added to your table, you can query expression columns in Select statements or

examine them with standard ADO.NET code just like static columns Expression columns are not calculated until you attempt to access them If there is anything wrong with the expres-sion, such as including references to non-existent columns, the code accessing the column will throw an exception