1. SQL Integrity Constraints

2. 421B: Database Systems - Integrity Constraints 2 Integrity Constraints (Review) q An IC describes conditions that every legal instance of a relation must satisfy. I Inserts/deletes/updates that violate IC’s are disallowed. I Can be used to ensure application semantics (e.g., sid is a key), or prevent inconsistencies (e.g., sname has to be a string, age must be < 200) q Commercial systems allow much more “fine- tuning” of constraints than do the modeling languages we learned so far

3. 421B: Database Systems - Integrity Constraints 3 Types of Constraints q Covered so far I Domain Constraints (data type, UNIQUE, NOT NULL) I Primary key constraints I Foreign key constraints (= referential integrity) q General constraints I Attribute- and tuple-based checks l Constraints within a relation l Updates/inserts that violate constraints are rejected. I SQL2 Assertions: global constraints l Constraints can cover several relations l All modifications that violate constraint are rejected l Many systems do not support Assertions I SQL3 Triggers l Upon user specified modifications, checks are performed and adequate reactions are executed

4. 421B: Database Systems - Integrity Constraints 4 Attribute-Based Checks q If a condition must hold for specific attribute: CHECK CREATE TABLE Sailors ( sid INTEGER PRIMARY KEY NOT NULL, sname VARCHAR(20), rating INTEGER CHECK(rating > 0 AND rating < 11), age INTEGER) q Condition is checked only when the associated attribute changes (i.e., an insert or update, but not delete!) q If condition is violated the system rejects the modification  In SQL condition can be anything that could follow WHERE clause I including subqueries I sname VARCHAR(20) CHECK (sname NOT IN (SELECT name FROM forbidden)) q DB2 allows very restricted attribute-based check (no subqueries, no reference to other attributes, …)

5. 421B: Database Systems - Integrity Constraints 5 Tuple-Based Checks q If a condition covers several attributes CREATE TABLE Sailors ( sid INTEGER PRIMARY KEY NOT NULL, name VARCHAR(20), rating INTEGER, age INTEGER, CHECK (rating 18))  Checked upon each update and insert  SQL99 allows subqueries but many databases don’t

6. 421B: Database Systems - Integrity Constraints 6 Naming constraints q Problem of previous examples: I what if constraints change (e.g., we want to increase rating constraint to (rating 18) q Solution: name constraints: CREATE TABLE Sailors ( sid INT, name VARCHAR(20), rating INT CONSTRAINT rat CHECK (rating > 0 AND rating < 11), age INT, CONSTRAINT pk PRIMARY KEY (sid), CONSTRAINT ratage CHECK (rating 18)) q This allows us to drop and recreate them later on ALTER TABLE Sailors DROP CONSTRAINT ratage ALTER TABLE SAILORS ADD CONSTRAINT ratage CHECK (rating 18)

7. 421B: Database Systems - Integrity Constraints 7 User defined Types  User can define new types from base types CREATE DISTINCT TYPE ratingval AS SMALLINT WITH COMPARISONS q The new types can be used for attribute definitions CREATE TABLE Sailors ( sid INTEGER PRIMARY KEY, name VARCHAR(20), rating ratingval, age SMALLINTEGER)

8. 421B: Database Systems - Integrity Constraints 8 Comparison with user defined types q With Comparisons: I we can compare two attributes that are of the type ratingval SELECT S1.sid, S2.sid FROM Sailors S1, Sailors S2 WHERE S1.sid < S2.sid AND S1.rating = S2.rating q We can’t compare attributes with different type deriving from the same base type I Incorrect: SELECT sid FROM Sailors WHERE rating > age

9. 421B: Database Systems - Integrity Constraints 9 Assertions q An assertion is a predicate expressing a condition that we wish the database always to satisfy. q Syntax: I CREATE ASSERTION ass-name CHECK (condition)  Reference to relations and attributes through SELECT statements CREATE ASSERTION smallClub CHECK ((SELECT COUNT (S.sid) FROM Sailors S) + (SELECT COUNT (B.bid) FROM Boats B) < 100) q Is checked whenever any of the relations involved is modified (delete/update/insert) q Especially useful if constraint covers more than one relation CREATE TABLE Sailors (… CHECK ((SELECT COUNT (S.sid) FROM Sailors S) + (SELECT COUNT (B.bid) FROM Boats B) < 100)) Here, constraint is checked only when Sailors are included or change sid, but not, when Boats changes! If bigClub (> 100), constraint should also be checked upon deletion; not done with CHECK

10. 421B: Database Systems - Integrity Constraints 10 Comparison CHECK/ASSERTION q When activated I Attribute based checks: on insert of tuple or update of attribute I Tuple based checks: on insert or update of tuple I Assertion: on insert or update or delete of tuple in any involving relation q Constraint Guarantee I Attribute/tuple based checks: no, if containing subquery I Assertion: yes. q Costs: I Attribute/tuple: only evaluate constraint for tuple that is inserted/updated (cheap) I Assertion: evaluate whenever one of the involving relations changes (expensive)

11. 421B: Database Systems - Integrity Constraints 11 Further Assertion Example CREATE ASSERTION rating-constraint CHECK (NOT EXISTS (SELECT * FROM Reserves R, Sailors S, Boats B WHERE R.sid = S.sid AND B.bid = R.bid AND S.rating < 7 AND B.boattype = ‘luxury’))

12. 421B: Database Systems - Integrity Constraints 12 Triggers q Trigger: procedure that starts automatically if specified changes occur to the DBMS q Three parts: I Event (activates the trigger): usually insert/update/delete I Condition (tests whether the trigger should run) I Action (what happens if the trigger runs) q Difference to Assertions and Checks: I When it is activated (the event) l Assertions etc.: upon any delete/update/insert l Triggers: user specifies the events that should fire the trigger: e.g., update of a specific attribute I What is done (the action) l Assertion etc.: reject modification l Triggers: Specify any action (sequence of SQL statements or reject) q Trigger introduced into SQL in 99, but many database systems supported it earlier: Syntax between DBS can vary considerably

13. 421B: Database Systems - Integrity Constraints 13 Triggers: Example (SQL3) q Statement Level Trigger CREATE TRIGGER includeintoWorks_in AFTER INSERT ON Employees REFERENCING NEW_TABLE AS NewEmployees FOR EACH STATEMENT INSERT INTO StatisticsTable(ModTable, ModType, Count) SELECT ‘Employees’, ‘INSERT’, COUNT(*) FROM NewEmployees q Row Level Trigger CREATE TRIGGER salaryIncrease AFTER UPDATE OF salary on Employees REFERENCING OLD AS o, NEW AS n FOR EACH ROW WHEN (n.salary > 1.1 * o.salary) UPDATE Employees SET salary = 1.1 * o.salary WHERE eid = n.eid

14. 421B: Database Systems - Integrity Constraints 14 Components of Triggers q Action can be executed before, after or instead of the triggering event.  AFTER UPDATE OF salary ON Employees (“of salary” optional) I BEFORE INSERT ON Employees I INSTEAD OF DELETE ON Employees q Action can refer to both the old and new values of tuples that were modified in the triggering event.  Row-level trigger: OLD/NEW references to old/new tuple l Old makes no sense in an insert, new makes no sense in a delete op.  Statement-level trigger: OLD_TABLE / NEW_TABLE  You can rename with the REFERENCING command  Condition in WHEN clause. Action is only executed if condition holds q Action performed I Row-level: once for each tuple changed in the triggering event I Statement-level: once for all tuples changed in the triggering event q Statement level can be more efficient if many tuples affected

15. 421B: Database Systems - Integrity Constraints 15 More on Triggers q Activating before event can serve as extra constraints. CREATE TRIGGER setnull-trigger BEFORE UPDATE ON r REFERENCING NEW AS nrow FOR EACH ROW WHEN nrow.phone-number = ‘ ‘ SET nrow.phone-number = null q Action can also be an error: rollback CREATE TRIGGER salaryIncrease AFTER UPDATE OF salary on Employees REFERENCING OLD AS o, NEW AS n FOR EACH ROW WHEN (n.salary > 1.1 * o.salary) SIGNAL SQLSTATE ‘120’ SET MESSAGE_TEXT=‘Salary increase above 10%’

16. 421B: Database Systems - Integrity Constraints 16 Several Actions q Use begin/end to encapsulate more than one action FOR EACH ROW/STATEMENT WHEN … BEGIN do 1thing; do 2nd thing END

17. 421B: Database Systems - Integrity Constraints 17 External World Actions q We sometimes require external world actions to be triggered on a database update I E.g. re-ordering an item whose quantity in a warehouse has become small, or turning on an alarm light, q Triggers cannot directly implement external-world actions, BUT I Triggers can be used to record actions-to-be-taken (e.g., in extra table) I Have an external process that repeatedly scans the table, carries out external-world actions and deletes action from table q E.g. Suppose a warehouse has the following tables I inventory(item, level): How much of each item is in the warehouse I minlevel(item, level) : What is the minimum desired level of each item I reorder(item, amount): What quantity should we re-order at a time I orders(item, amount) : Orders to be placed (read by external process)

18. 421B: Database Systems - Integrity Constraints 18 External Actions CREATE TRIGGER reorder-trigger AFTER UPDATE OF level ON inventory REFERENCING OLD ROW AS orow, NEW ROW AS nrow FOR EACH ROW WHEN nrow.level <= (SELECT level FROM minlevel WHERE minlevel.item = orow.item) AND orow.level > (SELECT level FROM minlevel WHERE minlevel.item = orow.item) BEGIN INSERT INTO orders (SELECT item, amount FROM reorder WHERE reorder.item = orow.item) END

19. 421B: Database Systems - Integrity Constraints 19 DB2 specifics q For each row q For each statement q Begin atomic … end q Rollback work --> signal sqlstate ….

20. 421B: Database Systems - Integrity Constraints 20 Other trigger use q Triggers are used for tasks such as I maintaining summary data (e.g. total salary of each departm.) I Replicating databases by recording changes to special relations (called change or delta relations) and having a separate process that applies the changes over to a replica q There are other ways of doing these now: I Databases today provide built in materialized view facilities to maintain summary data I Databases provide built-in support for replication q Encapsulation facilities can be used instead of triggers in many cases I Define methods to update fields I Carry out actions as part of the update methods instead of through a trigger

21. 421B: Database Systems - Integrity Constraints 21 Application vs. Trigger q Pro Trigger I Trigger defined at database design and design expert knows about constraints I Despite different application constraints, database consistency maintained I speed q Cons Trigger I If there are several triggers on same update things can become messy (what if trigger action triggers another trigger….) I If important application semantic implemented as triggers, then split of application logic l Part in application program / part as trigger l difficult to manage