Chapter 7 Constraints and Triggers

1. Chapter 7 Constraints and Triggers Spring 2011 Instructor: Hassan Khosravi

2. SQL: Constraints and Triggers • Certain properties we’d like our database to hold • Modification of the database may break these properties • Data entry may have errors • Build handlers into the database definition

3. SQL: Constraints and Triggers • (integrity) constraints (static) • Constrain allowable database states • Triggers (Dynamic) • Monitor database changes • Check conditions and initiate conditions

4. (integrity) Constraints • Impose restrictions on allowable data beyond those imposed by structure and types • Examples on university database • 0 < gpa < 4.0 • Enrollment < 50,000 • Decision attribute: ‘y’ or ‘n’ • Major = ‘CS’  decision = null • sizeHS < 200  not addmittedenr > 5000 • Why use integrity constraints • Data-entry error (insert) • gpa in range • Correctness criteria (update) • Enforce consistency • Referenced tuples • Tell system about your data

5. Classification of Integrity Constraints • Non- null • Keys • Uniqueness • Referential integrities (foreign key) • Attribute-based • Constraining values in attributes • Tuples-based • How values in different tuples should correlate • General assertions

6. Declaration and enforcing Constraints • Declaration • With original schema when tables are declared • Once you have a running database • Enforcement • Check after every dangerous modification • Changing major we don’t need to check the gpa constraint • Deferred constraint checking • We may do some modifications that would raise errors • But after we have done all the modifications it should be ok • Check once some modifications are done (transaction)

7. Triggers • Event-Condition-Action rules • When event occurs, check condition, if true, then do action • Example • Enrolllment > 75000  reject all applications • If application with gpa > 3.95  accept automatically • Update sizehs to be > 7000  change to wrong and raise error • Why use triggers? • Move codes from application to DBMS • Enforce constraints • Some of the assertions and checks are not implemented in some DBMS • Triggers could not only detect the problem, they can also solve it

8. Non-null constraints • Defining that a specific attribute in a specific table can not take the value of null • Create table A(A1int, A2 int not null, A3 text) • Examples

9. Key Constraints • The primary key of the tables has to be unique • Create table A (A1int primary key, A2 int, A3 text) • Create table A (A1int, A2 int, A3 text) primary key A1, A2)) • You can also define other combination of attributes to be unique ( without declaring them as key) • Create table A (A1int primary key, A2 int, A3 text unique) • Create table A (A1int primary key, A2 int, A3 text , unique(A1, A2)) • Examples

10. Attribute base check constraints • Constraints on the attributes to have specific ranges or types • Create table A (A1int, A2 int, A3 text, check (A2 >0)) • Examples

11. Assertions • Assertions are very strong they are checks done over all tables at the same time. (assertions are generally not implemented in current dbms) • Create assertion name check () • Example

12. Referential integrity • Referential integrity is integrity of references. • No dangling pointers • What would referential integrity from S.sid to A.sid mean?

13. Example

14. Example

15. Referential integrity

16. Referential integrity

19. Restrict

20. Set Null

21. Cascade

22. Update

23. Referential Integrity • Examples

24. Triggers • Event-Condition-Action rules • When event occurs, check condition, if true, then do action • Example • Enrolllment > 75000  reject all applications • If application with gpa > 3.95  accept automatically • Update sizehs to be > 7000  change to wrong and raise error

25. Triggers

26. Triggers Using Each Row

27. Triggers using table statement

28. Tricky issues with Triggers • Row-Level vs. Statement-level • Which one do you use? • Multiple triggers activated at the same time • For example a delete command triggers multiple triggers • Which one do you run first? • Chaining trigger action • Self triggering • Cycles Trigger A Trigger B Trigger C Trigger A • Nested triggers • Implementations are significantly different in different systems • Tricky example

29. Trigger Examples • For each student with GPA between 3.3 and 3.6, make them apply to geology in Stanford and biology in MIT

30. Trigger Examples • Implement referential integrity using triggers. • Delete all applications of a student from apply table after the student is deleted in the student table

31. Trigger Examples • Implement referential integrity using triggers on cascade. • Update the name of all colleges in the apply table if the name of the college is updated in the college table

32. Trigger Examples • Implement primary key using triggers. • Before inserting into table college check if the college exists. Raise an error if you get a duplicate • Example

33. Trigger Examples • Once a college has received 10 applications, add “-Done” to the end of the name of the college • Example of chains in triggers

34. Trigger Examples • If the size of the sizehk <100 or sizehk> 5000, then assume that its an error and ignore • Example considering before and after insert in chaining

35. Trigger Examples • Automatically accept students applying to Berkeley that have GPA > 3.7 and are coming from a highschool > 1200

36. Trigger Examples • Once a college passes a thresh-hold of 16000 applicants, delete new EE applicant and set all the decisions for all majors to undecided

37. More Trigger Examples (self triggers) • By default this may get triggered only once. • Recursive_trigger = on

38. More Trigger Examples (cycles) Example

39. More Trigger Examples (Conflicts) • What happens when you have multiple triggers being triggered at the same time

40. More Trigger Examples (Nested Triggers) • What order are triggers carried out in you have nested triggers • behaves like imperative programming

41. End of Chapter 7