lecture-03 accessing the right data values ado.net

7/28/2019 Lecture-03 Accessing the Right Data Values ADO.net

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Modern Programming Language

Lecture-03


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Accessing the Right Data Values

Agenda Find items in a DataTable by primary key

Search for DataRow instances using standardquery statements

Obtain a set of DataRow objects sorted by one ormore columns

Add expression columns to a DataTable thatpresent calculated values


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Queryingand Sorting Data

Although adding records to a DataTable is

important, the real value of ADO.NET lies ingetting those records back out in a variety of ways.

you can scan through every row in the table,checking each record as you encounter it to see

whether it matches your search limits, theDataTable class already includes features that willlet you select just those DataRow instances thatmatch a selection rule.


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Finding Rows by Primary Key To locate a row based on its primary key, use the

tables Rows.Find method. For tables with single-column keys, pass this method a key value of theappropriate data type.

For multi-column keys, pass the key components as

an array. Find returns a single DataRow instance forthe matching row.

' ----- 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

The method throws an exception if you apply it to tables with no

defined primary key.


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Example

Dim dr As DataRow = CustomerAccounts.Rows.Find(Me.TextBox1.Text)

Dim ds As DataTable = CustomerAccounts.Clone()

ds.ImportRow(dr)

DGV.DataSource = ds


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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, butuseful data analysis typically involves searching acrossmany of a tables columns and returning all possiblematches. To provide this functionality, the DataTableclass includes the Select method.

You pass the Select method a string that contains theselection criteria. When successful, the method returnsan array of matching DataRow instances from thetable.

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


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Example

Dim dr() As DataRow = dt.Select("ID


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Example-2 Search for String values

Dim dr() As DataRow = dt.Select("Name LIKE '" & Me.TextBox1.Text.Trim & "%'")Dim dt2 As Data.DataTable = dt.Clone

Dim r As DataRow

For Each r In dr

dt2.ImportRow(r)

Next

DataGridView1.DataSource = dt2

Performing Case-Sensitive

someTable.CaseSensitive = True

Sort the The result:

Dim dr() As DataRow = dt.Select("ID


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Using Expression Columns The DataTable class also supports expression columns,

fields that expose a calculated result based on the data in

other row columns.

For instance, if your table of orders includes a Subtotalcolumn and a Tax column, you could add an expressioncolumn named Total that calculated the sum of Subtotal

and Tax. ' ----- Syntax using a DataColumn object.

Dim orderTotal As New DataColumn

orderTotal.ColumnName = "Total"

orderTotal.DataType = GetType(Decimal)

orderTotal.Expression = Balance1+ ISNULL(Balance2, 0)"

someTable.Columns.Add(orderTotal)

' ----- Syntax using Add arguments only.

someTable.Columns.Add("Total", GetType(Decimal), Balance1+ ISNULL(Balance2, 0)" )


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Questions???

lecture-03 accessing the right data values ado.net

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