Tài liệu Adding restrictions to datatable and datacolumn objects phần 2 pptx

The following example uses the Add method to add a primary key constraint to the Products DataTable: myDataSet.Tables["Orders"].Constraints.Add "Primary key constraint", myDataSet.Tabl

• constraintName is the name you want to assign to your constraint

• isPrimaryKey indicates whether the constraint is a primary key constraint or just a regular unique constraint

The following example uses the Add() method to add a primary key constraint to the Products DataTable:

myDataSet.Tables["Orders"].Constraints.Add(

"Primary key constraint",

myDataSet.Tables["Orders"].Columns["OrderID"],

true

);

This example does the same thing as the previous example that added the primary key constraint using the PrimaryKey property Notice the last parameter to the Add() method

is set to true, which indicates the constraint is for a primary key

Just as an aside, if you have a column that isn't a primary key but is unique, you can add a UniqueConstraint object to the ConstraintsCollection For example:

UniqueConstraint myUC =

new UniqueConstraint(myDataTable.Columns["myColumn"]);

myDataTable.Constraints.Add(myUC);

Adding a Primary Key to the OrderDetails DataTable

Let's consider an example of setting the PrimaryKey property for the Order Details

DataTable The primary for the Order Details table is made up of the OrderID and

ProductID columns, and the following example sets the PrimaryKey property of the Order Details DataTable to these two columns:

myDataSet.Tables["Order Details"].PrimaryKey =

new DataColumn[]

{

myDataSet.Tables["Order Details"].Columns["OrderID"],

myDataSet.Tables["Order Details"].Columns["ProductID"]

};

The following example uses the Add() method to do the same thing:

myDataSet.Tables["Order Details"].Constraints.Add(

"Primary key constraint",

new DataColumn[]

{

myDataSet.Tables["Order Details"].Columns["OrderID"],

myDataSet.Tables["Order Details"].Columns["ProductID"]

},

true

);

One thing to keep in mind when adding constraints to a DataTable is that it knows only about the rows you store in it; it doesn't know about any other rows stored in the actual database table To see why this is an issue, consider the following scenario that involves primary keys:

1 You add a primary key constraint to a DataTable

2 You retrieve a subset of the rows from a database table and store them in your

DataTable

3 You add a new DataRow to your DataTable with a primary key value not used in the subset of rows retrieved into your DataTable in the previous step-but that

primary key value is already used in a row in the database table Your new

DataRow is added without any problem to the DataTable even though you added a primary key constraint to your DataTable in step 1 Your new DataRow is added

successfully because the DataTable knows only about the rows stored in it, not the

other rows stored in the database table that were not retrieved in step 2

4 You attempt to push the new DataRow to the database, but you get a SqlException that states you've violated the primary key constraint in the database table This is because a row in the database table already uses the primary key value

You need to keep this issue in mind when adding rows to a DataTable, which you'll see how to do shortly

That wraps up adding the primary key constraints to the DataTable objects Next, you'll see how to add foreign key constraints

Adding Foreign Key Constraints to the Order Details DataTable

In this section, you'll see how to add a foreign key constraint to the Order Details

DataTable To do this, you use the Add() method through the Constraints property of the DataTable

The following example adds a foreign key constraint from the OrderID DataColumn of the Order Details DataTable to the OrderID DataColumn of the Orders DataTable:

ForeignKeyConstraint myFKC = new ForeignKeyConstraint(

myDataSet.Tables["Orders"].Columns["OrderID"],

myDataSet.Tables["Order Details"].Columns["OrderID"]

);

myDataSet.Tables["Order Details"].Constraints.Add(myFKC);

Note Notice that the parent DataColumn (OrderID of Orders) is specified before the child DataColumn (OrderID of Order Details)

The next example adds a foreign key constraint from the ProductID DataColumn of the Order Details DataTable to the ProductID DataColumn of the Products DataTable:

myDataSet.Tables["Order Details"].Constraints.Add(

"Foreign key constraint to ProductID DataColumn of the " +

"Products DataTable",

myDataSet.Tables["Order Details"].Columns["ProductID"],

myDataSet.Tables["Products"].Columns["ProductID"]

);

That wraps up adding constraints to the DataTable objects Next, you'll see how to add restrictions to DataColumn objects

Adding Restrictions to DataColumn Objects

In this section, you'll see how to add restrictions to the DataColumn objects stored in a DataTable Specifically, you'll see how to set the AllowDBNull, AutoIncrement,

AutoIncrementSeed, AutoIncrementStep, ReadOnly, and Unique properties of the

ProductID DataColumn of the Products DataTable You'll also see how to set the

MaxLength property of the ProductName DataColumn of the Products DataTable

The ProductID column of the Products database table is an identity column The seed is the initial value and the step is the increment added to the last number and they are both set to 1 for ProductID The ProductID identity values are therefore 1, 2, 3, and so on

Tip When you set the AutoIncrementSeed and AutoIncrementStep properties for a

DataColumn that corresponds to a database identity column, you should always set them both to -1 That way, when you call the Fill() method, ADO.NET will

automatically figure out what values to set the AutoIncrementSeed and

AutoIncrementStep to, based on the values retrieved from the database, and you don't

have to figure out these values yourself

The following code sets the properties of the ProductID DataColumn:

DataColumn productIDDataColumn =

myDataSet.Tables["Products"].Columns["ProductID"];

productIDDataColumn.AllowDBNull = false;

productIDDataColumn.AutoIncrement = true;

productIDDataColumn.AutoIncrementSeed = -1;

productIDDataColumn.AutoIncrementStep = -1;

productIDDataColumn.ReadOnly = true;

productIDDataColumn.Unique = true;

The next example sets the MaxLength property of the ProductName DataColumn to 40 This stops you from setting the column value for ProductName to a string greater than 40 characters in length:

myDataSet.Tables["Products"].Columns["ProductName"].MaxLength = 40;

Listing 11.1 uses the code examples shown in this section and the previous one Notice this program also displays the ColumnName and DataType properties of the DataColumn objects in each DataTable The ColumnName property contains the name of the

DataColumn, and the DataType contains the NET data type used to represent the column value stored in the DataColumn

Listing 11.1: ADDRESTRICTIONS.CS

/*

AddRestrictions.cs illustrates how to add constraints to

DataTable objects and add restrictions to DataColumn objects

*/

using System;

using System.Data;

using System.Data.SqlClient;

class AddRestrictions

{

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.Fill(myDataSet);

mySqlConnection.Close();

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

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

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

// set the PrimaryKey property for the Products DataTable

// to the ProductID column

DataTable productsDataTable = myDataSet.Tables["Products"]; DataColumn[] productsPrimaryKey =

new DataColumn[]

{

productsDataTable.Columns["ProductID"]

};

productsDataTable.PrimaryKey = productsPrimaryKey;

// set the PrimaryKey property for the Orders DataTable

// to the OrderID column

myDataSet.Tables["Orders"].PrimaryKey =

new DataColumn[]

{

myDataSet.Tables["Orders"].Columns["OrderID"]

};

// set the PrimaryKey property for the Order Details DataTable // to the OrderID and ProductID columns

myDataSet.Tables["Order Details"].Constraints.Add(

"Primary key constraint on the OrderID and ProductID columns", new DataColumn[]

{

myDataSet.Tables["Order Details"].Columns["OrderID"], myDataSet.Tables["Order Details"].Columns["ProductID"] },

true

);

// add a foreign key constraint on the OrderID column

// of Order Details to the OrderID column of Orders

ForeignKeyConstraint myFKC = new ForeignKeyConstraint(

myDataSet.Tables["Orders"].Columns["OrderID"],

myDataSet.Tables["Order Details"].Columns["OrderID"]

);

myDataSet.Tables["Order Details"].Constraints.Add(myFKC);

// add a foreign key constraint on the ProductID column

// of Order Details to the ProductID column of Products

myDataSet.Tables["Order Details"].Constraints.Add(

"Foreign key constraint to ProductID DataColumn of the " +

"Products DataTable",

myDataSet.Tables["Products"].Columns["ProductID"],

myDataSet.Tables["Order Details"].Columns["ProductID"]

);

// set the AllowDBNull, AutoIncrement, AutoIncrementSeed,

// AutoIncrementStep, ReadOnly, and Unique properties for

// the ProductID DataColumn of the Products DataTable

DataColumn productIDDataColumn =

myDataSet.Tables["Products"].Columns["ProductID"];

productIDDataColumn.AllowDBNull = false;

productIDDataColumn.AutoIncrement = true;

productIDDataColumn.AutoIncrementSeed = -1;

productIDDataColumn.AutoIncrementStep = -1;

productIDDataColumn.ReadOnly = true;

productIDDataColumn.Unique = true;

// set the MaxLength property for the ProductName DataColumn

// of the Products DataTable

myDataSet.Tables["Products"].Columns["ProductName"].MaxLength = 40;

// 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);

}

// 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:

Reading from the Products DataTable:

myPrimaryKey = ProductID

myDataColumn.ColumnName = ProductID

myDataColumn.DataType = System.Int32

myDataColumn.AllowDBNull = False

myDataColumn.AutoIncrement = True

myDataColumn.AutoIncrementSeed = -1

myDataColumn.AutoIncrementStep = -1

myDataColumn.MaxLength = -1

myDataColumn.ReadOnly = True

myDataColumn.Unique = True

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

myDataColumn.AutoIncrement = False

myDataColumn.AutoIncrementSeed = 0

myDataColumn.AutoIncrementStep = 1

myDataColumn.MaxLength = 40

myDataColumn.ReadOnly = False

myDataColumn.Unique = False

Reading from the Orders DataTable: