2. SQL Server 7.0 Maintaining Referential Integrity

3. SQL Server is a R DBMS Relationships between entities in the data model. Entities are implemented as Tables. A mechanism ensures that Referential Integrity is not violated.

4. Mechanisms to enforce Referential Integrity, Data Integrity Data integrity is enforced by constraints, triggers, rules, and defaults. We will focus on constraints and triggers. Constraints are: –Simple. –Proactive. –Query optimizer uses them. Triggers: –Complex logic. –Checking data in other tables/cascade changes. –Reactive / Incur more overhead.

5. Entity Integrity PRIMARY KEY constraint. –Creates a unique index. –Disallows Nulls. UNIQUE constraint. –Creates a unique index. –Allows Nulls. UNIQUE index.

6. FOREIGN KEY FOREIGN KEY constraint REFERENCES primary table’s PRIMARY KEY. It is recommended not to allow changes to the PRIMARY KEY, this may lead to the complex enforcement approach - Triggers.

7. Relationship between two tables A typical Primary / Secondary table’s scenario. Example: Orders and OrderDetails.

8. Relationship in a Self-Referencing table A typical self- referencing table scenario. Example: Employees table, where each employee has a manager in charge; the manager is an employee himself.

9. Four Referential Integrity rules Deleting a row from PrimaryTable is not allowed if there are related rows in SecondaryTable. An update to PrimaryTable.col1 is not allowed if there are related rows in SecondaryTable. An insert of a row into SecondaryTable is not allowed if there is no related row in PrimaryTable. An update to SecondaryTable.col1 is not allowed if there is no related row in PrimaryTable.

10. FOREIGN KEY and REFERENCES constraint The best way to enforce all those rules is to create a FOREIGN KEY and REFERENCES constraint, as the following statement shows: ALTER TABLE SecondaryTable ADD CONSTRAINT FK_SecondaryTable_PrimaryTable FOREIGN KEY(col1) REFERENCES PrimaryTable(col1) We will use the same constraint to enforce Referential Integrity in the Self-Referencing table, as the following statement shows: ALTER TABLE SelfRefTable ADD CONSTRAINT FK_SelfRefTable_SelfRefTable FOREIGN KEY(col2) REFERENCES SelfRefTable(col1)

11. Triggers and Cascading changes Things get complicated if we need to allow changes to PrimaryTable and cascade those changes to SecondaryTable. We need to decide whether to allow cascading DELETE and/or UPDATE operations. Constraints can not be used in this case.

12. Choosing the right Referential Integrity mechanism

13. Cascading DELETE Operations The following trigger achieves cascading DELETE operations from the PrimaryTable to the SecondaryTable: CREATE TRIGGER dbo.TRG_cascade_delete ON PrimaryTable FOR DELETE AS DELETE FROM SecTbl FROM SecondaryTable SecTbl INNER JOIN deleted d ON SecTbl.col1 = d.col1

14. Trigger Is a special kind of a stored procedure. Is invoked automatically for individual INSERT, UPDATE, DELETE operation, or a combination of those operations. Creates special areas in cache where it keeps the modified data, before and after the change. Is an implicit transaction. Now has 32 nesting levels. Can define more than one trigger of the same type on the same table.

15. Cascade DELETE example

16. Preventing a DELETE operation If we decide not to cascade DELETE operations we need to enforce an invalid DELETE operation: CREATE TRIGGER dbo.TRG_prevent_delete ON PrimaryTable FOR DELETE AS IF EXISTS (SELECT * FROM SecondaryTable SecTbl INNER JOIN deleted d ON SecTbl.col1 = d.col1) BEGIN RAISERROR('PrimaryTable has related rows in SecondaryTable. TRANSACTION rolled back.', 10, 1) ROLLBACK TRANSACTION END

17. Cascading UPDATE operations CREATE TRIGGER dbo.TRG_cascade_update ON PrimaryTable FOR UPDATE AS DECLARE @numrows int SET @numrows = @@rowcount IF UPDATE(col1) IF @numrows = 1 UPDATE SecTbl SET col1 = (SELECT col1 FROM inserted) FROM deleted d INNER JOIN SecondaryTable SecTbl ON d.col1 = SecTbl.col1 ELSE IF @numrows > 1 BEGIN RAISERROR('Updates to more than one row in PrimaryTable are not allowed. TRANSACTION rolled back.', 10, 1) ROLLBACK TRANSACTION END

18. Cascade UPDATE example

19. Preventing invalid updates CREATE TRIGGER dbo.TRG_prevent_update ON PrimaryTable FOR UPDATE AS IF EXISTS (SELECT * FROM SecondaryTable SecTbl INNER JOIN deleted d ON SecTbl.col1 = d.col1) BEGIN RAISERROR('PrimaryTable has related rows in SecondaryTable. TRANSACTION rolled back.', 10, 1) ROLLBACK TRANSACTION END

20. Preventing invalid changes to the SecondaryTable CREATE TRIGGER dbo.TRG_prevent_insupd ON SecondaryTable FOR INSERT, UPDATE AS DECLARE @numrows int SET @numrows = @@rowcount IF @numrows <> (SELECT COUNT(*) FROM PrimaryTable PrmTbl INNER JOIN inserted i ON PrmTbl.col1 = i.col1) BEGIN RAISERROR('Result rows in SecondaryTable do not have related rows in PrimaryTable. TRANSACTION rolled back.', 10, 1) ROLLBACK TRANSACTION END

21. Encapsulating the logic sp_CreateRelationship [ @primary_table =] ‘primary table name’,[ @secondary_table =] ‘secondary table name’,[ @primary_col1 =] ‘primary column name_n’, [,…16],[ @secondary_col1 =] ‘secondary column name_n’, [,…16] [,[ @cascade_updates =] 'true' | 'false'] [,[ @cascade_deletes =] 'true' | 'false'] Implementing the stored procedure is just a matter of getting the user’s input and combining it in the ALTER TABLE ADD CONSTRAINT or CREATE TRIGGER commands.

22. Summery A poorly designed database will lead to the use of complex, resource consuming mechanisms such as triggers. In most of the cases, a well-designed database will allow the use of the preferred mechanism – constraints. I presented deferent scenarios that need different approaches in implementing Referential Integrity.