Tài liệu Creating a UniqueConstraint Object ppt

Creating a UniqueConstraint Object In this section, you'll learn how to create a UniqueConstraint object.. The UniqueConstraint constructor is overloaded as follows: UniqueConstraintData

Creating a UniqueConstraint Object

In this section, you'll learn how to create a UniqueConstraint object The

UniqueConstraint constructor is overloaded as follows:

UniqueConstraint(DataColumn myDataColumn)

UniqueConstraint(DataColumn[] myDataColumns)

UniqueConstraint(DataColumn myDataColumn, bool isPrimaryKey)

UniqueConstraint(DataColumn[] myDataColumns, bool isPrimaryKey)

UniqueConstraint(string constraintName, DataColumn myDataColumn)

UniqueConstraint(string constraintName, DataColumn[] myDataColumns)

UniqueConstraint(string constraintName, DataColumn myDataColumn, bool

isPrimaryKey)

UniqueConstraint(string constraintName, DataColumn[] myDataColumns,

bool isPrimaryKey)

UniqueConstraint(string constraintName, string[] columnNames,

bool isPrimaryKey)

where

• myDataColumn and myDataColumns are the DataColumn objects that you want

to ensure are unique

• constraintName is the name you want to assign to the ConstraintName property of

your UniqueConstraint

• isPrimaryKey indicates whether your UniqueConstraint is for a primary key

Before creating and adding a UniqueConstraint to a DataTable, you first need a

DataTable The following example creates and populates two DataTable objects named customersDT and ordersDT (the ordersDT object will be used later in the section

"Creating a ForeignKeyConstraint Object"):

SqlCommand mySqlCommand = mySqlConnection.CreateCommand();

mySqlCommand.CommandText =

"SELECT CustomerID, CompanyName " +

"FROM Customers " +

"WHERE CustomerID = 'ALFKI'" +

"SELECT OrderID, CustomerID " +

"FROM Orders " +

"WHERE CustomerID = 'ALFKI';";

SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter();

mySqlDataAdapter.SelectCommand = mySqlCommand;

DataSet myDataSet = new DataSet();

mySqlConnection.Open();

mySqlDataAdapter.Fill(myDataSet);

mySqlConnection.Close();

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

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

DataTable customersDT = myDataSet.Tables["Customers"];

DataTable ordersDT = myDataSet.Tables["Orders"];

The following example creates a UniqueConstraint object on the CustomerID

DataColumn of the customersDT DataTable, and then adds this UniqueConstraint to customersDT; notice that the third parameter to the UniqueConstraint constructor is set to true, indicating that the constraint is for a primary key:

UniqueConstraint myUC =

new UniqueConstraint(

"UniqueConstraintCustomerID",

customersDT.Columns["CustomerID"],

true

);

customersDT.Constraints.Add(myUC);

Note To successfully add a UniqueConstraint to the DataColumn of a DataTable, the DataColumn value in each DataRow object in the DataTable must be unique

The final example retrieves the constraint just added to customersDT and displays its properties:

myUC =

(UniqueConstraint) customersDT.Constraints["UniqueConstraintCustomerID"];

Console.WriteLine("Columns:");

foreach (DataColumn myDataColumn in myUC.Columns)

{

Console.WriteLine("" + myDataColumn);

}

Console.WriteLine("myUC.ConstraintName = " + myUC.ConstraintName);

Console.WriteLine("myUC.IsPrimaryKey = " + myUC.IsPrimaryKey);

Console.WriteLine("myUC.Table = " + myUC.Table);

This example displays the following output:

Columns:

CustomerID

myUC.ConstraintName = UniqueConstraintCustomerID

myUC.IsPrimaryKey = True

myUC.Table = Customers

The IsPrimaryKey property is true because the constraint is for a primary key; this was specified in the constructor when the constraint was created earlier