DeleteRule in .NET

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Obviously, an object that has only a name and a container is of little use when it comes to enforcing integrity. In real applications, you will instantiate one of the objects that inherits from Constraint, ForeignKeyConstraint, or UniqueConstraint. The properties of the ForeignKeyConstraint object are shown in Table 7-10. This constraint represents the rules that are enforced when a parent-child relationship exists between tables (or between rows within a single table). Table 7-10: ForeignKeyConstraint Properties Property AcceptRejectRule Description Determines the action
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Table 7-10: ForeignKeyConstraint Properties Property Description that should take place when the AcceptChan ges method is called Columns The collection of child columns for the constraint The action that will take place when the row is deleted The collection of parent columns for the constraint The parent DataTable for the constraint Overrides the Constraint.T able property to return the child DataTable for the constraint The action that will take place when the row is updated
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The actions to take place to enforce integrity are maintained by three properties of the ForeignKeyConstraint: AcceptRejectRule, DeleteRule, and UpdateRule. The possible values of the AcceptRejectRule property are Cascade or None. The DeleteRule and UpdateRule properties can be set to any of the values shown in Table 711. Both properties have a default value of Cascade. Table 7-11: Action Rules Property Cascade Description Delete or update the
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Table 7-11: Action Rules Property Description related rows None Take no action on the related rules Set values in the related rows to their default values Set values in the related rows to Null
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SetDefault
SetNull
Add a ForeignKeyConstraint
Visual Basic .NET
1. In the code editor, select btnForeign in the ControlName list, and then select Click in the MethodName list. Visual Studio adds the Click event handler template to the code. 2. Add the following code to create the ForeignKeyConstraint: 3. Dim strMessage As String 4. Dim fkNew As System.Data.ForeignKeyConstraint 5. 6. With Me.dsUntyped 7. 8. 9. 10. 11. 12. 13. 14. 15. MessageBox.Show(strMessage) 16. Press F5 to run the application. 17. Click Foreign Key. The application adds the ForeignKeyConstraint and displays its name in a message box. strMessage = "The new constraint is called " strMessage &= .Tables("dtChild").Constraints(0).ConstraintName.ToString End With fkNew = New System.Data.ForeignKeyConstraint("NewFK", _ .Tables("dtMaster").Columns("MasterID"), _ .Tables("dtChild").Columns("MasterLink")) .Tables("dtChild").Constraints.Add(fkNew)
18. Close the application. Visual C# .NET 1. In the form designer, double-click the Foreign Key button. Visual Studio adds the Click event handler to the code window. 2. Add the following code to create the ForeignKeyConstraint: 3. 4. 5. 6. 7. 8. 9. 11. 12. strMessage = "The new constraint is called "; 13. strMessage += 14. ds.Tables["dtChild"].Constraints[0].ConstraintName.ToString(); MessageBox.Show(strMessage); 15. Press F5 to run the application. 16. Click Foreign Key. The application adds the ForeignKeyConstraint and displays its name in a message box. fkNew = new System.Data.ForeignKeyConstraint("NewFK", ds.Tables["dtMaster"].Columns["MasterID"], ds.Tables["dtChild"].Columns["MasterLink"]); string strMessage; System.Data.ForeignKeyConstraint fkNew; System.Data.DataSet ds = this.dsUntyped;
10. ds.Tables["dtChild"].Constraints.Add(fkNew);
17. Close the application. The UniqueConstraint ensures that the column or columns specified in its Columns property are unique within the table. Its structure is much simpler than a ForeignKeyConstraint, as shown in Table 7-12. Table 7-12: UniqueConstraint Properties Property Columns Description The array of columns affected by the constraint Indicates whether the constraint is on the primary key
IsPrimaryKey
Add a UniqueConstraint
Visual Basic .NET
1. In the code editor, select btnUnique in the ControlName list, and then select Click in the MethodName list. Visual Studio adds the Click event handler template to the code. 2. Add the following code to create the UniqueConstraint: 3. Dim strMessage As String 4. Dim ucNew As System.Data.UniqueConstraint 5. 6. With Me.dsUntyped.Tables("dtMaster") 7. 8. 9. 10. 11. 12. strMessage = "The new constraint is called " strMessage &= .Constraints("NewUnique").ConstraintName.ToString ucNew = New System.Data.UniqueConstraint("NewUnique", _ .Columns("MasterValue")) .Constraints.Add(ucNew)
13. 14.
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