Chapter 7 Constraints and Triggers Spring 2011 Instructor: Hassan Khosravi
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 7.2
SQL: Constraints and Triggers (integrity) constraints (static) Constrain allowable database states Triggers (Dynamic) Monitor database changes Check conditions and initiate conditions 7.3
(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 addmitted enr > 5000 Why use integrity constraints Data-entry error (insert) gpa in range Correctness criteria (update) Enforce consistency Referenced tuples Tell system about your data 7.4
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 7.5
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) 7.6
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 7.7
Non-null constraints Defining that a specific attribute in a specific table can not take the value of null Create table A(A 1 int, A 2 int not null, A 3 text) Examples 7.8
Key Constraints The primary key of the tables has to be unique Create table A (A 1 int primary key, A 2 int, A 3 text) Create table A (A 1 int, A 2 int, A 3 text) primary key A 1, A 2 )) You can also define other combination of attributes to be unique ( without declaring them as key) Create table A (A 1 int primary key, A 2 int, A 3 text unique) Create table A (A 1 int primary key, A 2 int, A 3 text, unique(a 1, A 2 )) Examples 7.9
Attribute base check constraints Constraints on the attributes to have specific ranges or types Create table A (A 1 int, A 2 int, A 3 text, check (A 2 >0)) Examples 7.10
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 7.11
Referential integrity Referential integrity is integrity of references. No dangling pointers What would referential integrity from S.sid to A.sid mean? 7.12
Example 7.13
Example 7.14
Referential integrity 7.15
Referential integrity 7.16
7.17
7.18
Restrict 7.19
Set Null 7.20
Cascade 7.21
Update 7.22
Referential Integrity Examples 7.23
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 7.24
Triggers 7.25
Triggers Using Each Row 7.26
Triggers using table statement 7.27
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 7.28
Trigger Examples For each student with GPA between 3.3 and 3.6, make them apply to geology in Stanford and biology in MIT 7.29
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 7.30
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 7.31
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 7.32
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 7.33
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 7.34
Trigger Examples Automatically accept students applying to Berkeley that have GPA > 3.7 and are coming from a highschool > 1200 7.35
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 7.36
More Trigger Examples (self triggers) By default this may get triggered only once. Recursive_trigger = on 7.37
More Trigger Examples (cycles) Example 7.38
More Trigger Examples (Conflicts) What happens when you have multiple triggers being triggered at the same time 7.39
More Trigger Examples (Nested Triggers) What order are triggers carried out in you have nested triggers behaves like imperative programming 7.40
END OF CHAPTER 7 7.41