Tài liệu Adding Restrictions to DataTable and DataColumn Objects phần 3 ppt

myDataColumn.MaxLength = -1 myDataColumn.ReadOnly = False myDataColumn.Unique = False Adding Restrictions by Calling the DataAdapter Object's FillSchema Method Instead of adding restric

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myPrimaryKey = OrderID

myDataColumn.ColumnName = OrderID myDataColumn.DataType = System.Int32 myDataColumn.AllowDBNull = False

myDataColumn.AutoIncrement = False myDataColumn.AutoIncrementSeed = 0 myDataColumn.AutoIncrementStep = 1 myDataColumn.MaxLength = -1

myDataColumn.ReadOnly = False

myDataColumn.Unique = True

Reading from the Order Details DataTable:

myPrimaryKey = ProductID

myDataColumn.Unique = False

myDataColumn.ColumnName = ProductID myDataColumn.DataType = System.Int32 myDataColumn.AllowDBNull = False

myDataColumn.ColumnName = UnitPrice myDataColumn.DataType = System.Decimal myDataColumn.AllowDBNull = True

myDataColumn.AutoIncrement = False myDataColumn.AutoIncrementSeed = 0 myDataColumn.AutoIncrementStep = 1

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myDataColumn.MaxLength = -1

Adding Restrictions by Calling the DataAdapter Object's FillSchema() Method

Instead of adding restrictions yourself, you can add them by calling the FillSchema() method of your DataAdapter The FillSchema() method does the following:

• Copies the schema information from the database

• Creates DataTable objects in your DataSet if they don't already exist

• Adds the constraints to the DataTable objects

• Sets the properties of the DataColumn objects appropriately

The properties of the DataColumn objects set by FillSchema() include the following:

• The DataColumn name-which is stored in the ColumnName property

• The DataColumn NET data type-which is stored in the DataType property

• The maximum length of a variable length data type-which is stored in the

MaxLength property

• Whether the DataColumn can accept a null value-which is stored in the

AllowDBNull property

• Whether the DataColumn value must be unique-which is stored in the Unique property

• Any auto-increment information-which is stored in the AutoIncrement,

AutoIncrementSeed, and AutoIncrementStep properties

The FillSchema() method will also determine whether the DataColumn is part of a

primary key and store that information in the PrimaryKey property of the DataTable

Warning FillSchema() does not automatically add ForeignKeyConstraint objects to the

DataTable objects Neither does it retrieve the actual rows from the database; it

retrieves only the schema information

The FillSchema() method is overloaded, with the most commonly used version of this method being the following:

DataTable[] FillSchema(DataSet myDataSet, SchemaType mySchemaType)

where mySchemaType specifies how you want to handle any existing schema mappings You set mySchemaType to one of the constants defined in the System.Data.SchemaType

enumeration Table 11.7 shows the constants defined in the SchemaType enumeration

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Table 11.7: SchemaType ENUMERATION MEMBERS

CONSTANT DESCRIPTION

Mapped Apply any existing table mappings to the incoming schema and configure

the DataSet with the transformed schema This is the constant you should typically use

Source Ignore any table mappings and configure the DataSet without any

transformations

Let's take a look at an example that contains a call to the FillSchema() method Notice the call uses the SchemaType.Mapped constant to apply any existing table mappings:

SqlCommand mySqlCommand = mySqlConnection.CreateCommand();

mySqlCommand.CommandText =

"SELECT ProductID, ProductName " +

"FROM Products;" +

"SELECT OrderID " +

"FROM Orders;" +

"SELECT OrderID, ProductID, UnitPrice " +

"FROM [Order Details];";

SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter();

mySqlDataAdapter.SelectCommand = mySqlCommand;

DataSet myDataSet = new DataSet();

mySqlConnection.Open();

mySqlDataAdapter.FillSchema(myDataSet, SchemaType.Mapped);

mySqlConnection.Close();

myDataSet.Tables["Table"].TableName = "Products";

myDataSet.Tables["Table1"].TableName = "Orders";

myDataSet.Tables["Table2"].TableName = "Order Details";

The call to FillSchema() copies the schema information from the Products, Orders, and Order Details tables to myDataSet, setting the PrimaryKey property of each DataTable and the properties of the DataColumn objects appropriately

Listing 11.2: FILLSCHEMA.CS

/*

FillSchema.cs illustrates how to read schema information

using the FillSchema() method of a DataAdapter object

*/

using System;

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using System.Data;

using System.Data.SqlClient;

class FillSchema

{

public static void Main()

{

SqlConnection mySqlConnection =

new SqlConnection(

"server=localhost;database=Northwind;uid=sa;pwd=sa"

);

SqlCommand mySqlCommand = mySqlConnection.CreateCommand(); mySqlCommand.CommandText =

"SELECT ProductID, ProductName " +

"FROM Products;" +

"SELECT OrderID " +

"FROM Orders;" +

"SELECT OrderID, ProductID, UnitPrice " +

"FROM [Order Details];";

SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter();

mySqlDataAdapter.SelectCommand = mySqlCommand;

DataSet myDataSet = new DataSet();

mySqlConnection.Open();

mySqlDataAdapter.FillSchema(myDataSet, SchemaType.Mapped); mySqlConnection.Close();

myDataSet.Tables["Table"].TableName = "Products";

myDataSet.Tables["Table1"].TableName = "Orders";

myDataSet.Tables["Table2"].TableName = "Order Details";

// display the details of the DataColumn objects for

// the DataTable objects

foreach (DataTable myDataTable in myDataSet.Tables)

{

Console.WriteLine("\n\nReading from the " +

myDataTable + "DataTable:\n");

// display the primary key

foreach (DataColumn myPrimaryKey in myDataTable.PrimaryKey) {

Console.WriteLine("myPrimaryKey = " + myPrimaryKey);

}

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// display the constraints

foreach (Constraint myConstraint in myDataTable.Constraints)

{

Console.WriteLine("myConstraint.IsPrimaryKey = " + ((UniqueConstraint) myConstraint).IsPrimaryKey);

foreach (DataColumn myDataColumn in ((UniqueConstraint)

myConstraint).Columns)

{

Console.WriteLine("myDataColumn.ColumnName = " +

myDataColumn.ColumnName);

}

// display some of the details for each column

foreach (DataColumn myDataColumn in myDataTable.Columns)

{

Console.WriteLine("\nmyDataColumn.ColumnName = " +

myDataColumn.ColumnName);

Console.WriteLine("myDataColumn.DataType = " +

myDataColumn.DataType);

Console.WriteLine("myDataColumn.AllowDBNull = " +

myDataColumn.AllowDBNull);

Console.WriteLine("myDataColumn.AutoIncrement = " +

myDataColumn.AutoIncrement);

Console.WriteLine("myDataColumn.AutoIncrementSeed = " +

myDataColumn.AutoIncrementSeed);

Console.WriteLine("myDataColumn.AutoIncrementStep = " +

myDataColumn.AutoIncrementStep);

Console.WriteLine("myDataColumn.MaxLength = " +

myDataColumn.MaxLength);

Console.WriteLine("myDataColumn.ReadOnly = " +

myDataColumn.ReadOnly);

Console.WriteLine("myDataColumn.Unique = " +

myDataColumn.Unique);

}

The output from this program is as follows:

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Reading from the Products DataTable:

myConstraint.IsPrimaryKey = True

myDataColumn.ColumnName = ProductID

myDataColumn.AutoIncrement = True

myDataColumn.AutoIncrementSeed = 0

myDataColumn.AutoIncrementStep = 1

myDataColumn.ReadOnly = True

myDataColumn.ColumnName = ProductName myDataColumn.DataType = System.String myDataColumn.AllowDBNull = False

myDataColumn.AutoIncrement = False

myDataColumn.MaxLength = 40

Reading from the Orders DataTable:

myDataColumn.ColumnName = OrderID

myDataColumn.AutoIncrement = True

myDataColumn.ReadOnly = True

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Reading from the Order Details DataTable:

myDataColumn.ColumnName = OrderID myDataColumn.ColumnName = ProductID

myDataColumn.ColumnName = UnitPrice myDataColumn.DataType = System.Decimal myDataColumn.AllowDBNull = False

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