Outerjoins, Constraints, Triggers

Transcription

1 Outerjoins, Constraints, Triggers Lecture #13 Autumn, 2001 Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 358

2 Outerjoin R S = R S with danging tupes padded with nus and incuded in the resut. A tupe is danging if it doesn't join with any other tupe. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 359

3 Danging Tupes R = A B S = B C R S = A B C NULL NULL 7 8 Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 360

4 Outerjoin in SQL2 A number of forms are provided.»can be used either stand-aone (in pace of a seect-from-where) or to define a reation in the FROM-cause. R NATURAL JOIN S R JOIN S ON condition e.g., condition: R:B = S:B R CROSS JOIN S R OUTER JOIN S Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 361

5 Outerjoin in SQL2 (II) The atter can be modified by:»optiona NATURAL in front of JOIN.»Optiona ON condition at end.»optiona LEFT, RIGHT, or FULL before OUTER. LEFT = pad danging tupes of R ony; RIGHT = pad danging tupes of S ony. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 362

6 Orace Outerjoin Orace has no such thing.»but parenthesized seect-from-where aowed in a FROM cause. Reay a way to define a view and use it in a singe query. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 363

7 Exampe Find the average over a bars of the maximum price the bar charges for a beer. Ses(bar, beer, price) SELECT AVG(maxPrice) FROM (SELECT bar, MAX(price) AS maxprice FROM Ses GROUP BY Bar); Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 364

8 Probem Can we express the outerjoin in Orace SQL as some more compicated expression? Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 365

9 Constraints Commercia reationa systems aow much more "fine-tuning" of constraints than do the modeing anguages we earned earier. In essence: SQL programming is used to describe constraints. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 366

10 Outine» 1. Primary key decarations (covered).» 2. Foreign-keys = referentia integrity constraints. E.g., if Ses mentions a beer, then we shoud be abe to find that beer in Beers.» 3. Attribute- and tupe-based checks = constraints within reations.» 4. SQL2 Assertions = goba constraints. Not found in Orace » 5. Orace Triggers. A substitute for assertions.» 6. SQL3 triggers and assertions. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 367

11 Foreign Keys In reation R a cause that "attribute A references S ( B )" says that whatever vaues appear in the A coumn of R must aso appear in the B coumn of reation S.»B must be decared the primary key for S. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 368

12 Exampe CREATE TABLE Beers ( name CHAR(20) PRIMARY KEY, manf CHAR(20) ); CREATE TABLE Ses ( bar CHAR(20), beer CHAR(20) REFERENCES Beers(name), price REAL ); Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 369

13 Aternative Add another eement decaring the foreign key, as: CREATE TABLE Ses ( bar CHAR(20), beer CHAR(20), price REAL, FOREIGN KEY beer REFERENCES Beers(name) ); Extra eement essentia if the foreign key is more than one attribute. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 370

14 What Happens When a Foreign Key Constraint is Vioated? Two ways:»1. Insert a Ses tupe referring to a nonexistent beer. Aways rejected.»2. Deete or update a Beers tupe that has a beer vaue some Ses tupes refer to. a) Defaut: reject. b) Cascade: Rippe changes to referring Ses tupe. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 371

15 Exampe Deete "Bud." Cascade deetes a Ses tupes that mention Bud. Update "Bud" "Budweiser." Change a Ses tupes with "Bud" in beer coumn to be "Budweiser." Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 372

16 What Happens When a Foreign Key Constraint is Vioated? (II) c) Set Nu : Change referring tupes to have NULL in referring components.»exampe Deete "Bud." Set-nu makes a Ses tupes with "Bud" in the beer component have NULL there. Update "Bud" "Budweiser." Same change. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 373

17 Seecting a Poicy Add ON [DELETE, UPDATE] [CASCADE, SET NULL] to decaration of foreign key. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 374

18 Seecting a Poicy (II) Exampe CREATE TABLE Ses ( bar CHAR(20), beer CHAR(20), price REAL, FOREIGN KEY beer REFERENCES Beers(name) ON DELETE SET NULL ON UPDATE CASCADE ); Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 375

19 Seecting a Poicy (III) "Correct" poicy is a design decision.»e.g., what does it mean if a beer goes away? What if a beer changes its name? Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 376

20 Attribute-Based Checks Foow an attribute by a condition that must hod for that attribute in each tupe of its reation. Form: CHECK (condition).» Condition may invove the checked attribute.» Other attributes and reations may be invoved, but ony in subqueries.» Orace 8.0.5: No subqueries aowed in condition. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 377

21 Attribute-Based Checks Condition is checked ony when the associated attribute changes (i.e., an insert or update occurs). Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 378

22 Exampe CREATE TABLE Ses ( bar CHAR(20), beer CHAR(20) CHECK( beer IN (SELECT name FROM Beers) ), price REAL CHECK( price <= 5.00 ) ); Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 379

23 Attribute-Based Checks (III) Check on beer is ike a foreign-key constraint, except»the check occurs ony when we add a tupe or change the beer in an existing tupe, not when we deete a tupe from Beers. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 380

24 Tupe-Based Checks Separate eement of tabe decaration. Form: ike attribute-based check. But condition can refer to any attribute of the reation.»or to other reations/attributes in subqueries.»again: Orace forbids the use of subqueries. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 381

25 Exampe Ony Joe's Bar can se beer for more than $5. CREATE TABLE Ses ( bar CHAR(20), beer CHAR(20), price REAL, CHECK(bar = 'Joe''s Bar' OR price <= 5.00) ); Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 382

26 Triggers Often caed event-condition-action rues. Event = a cass of changes in the DB, e.g., "insert into Beers." Condition = a test as in a where-cause for whether or not the trigger appies. Action = one or more SQL statements. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 383

27 Triggers (II) Orace version and SQL3 version; not in SQL2. Differ from checks or SQL2 assertions in that:»1. Event is programmabe, rather than impied by the kind of check.»2. Condition not avaiabe in checks. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 384

28 Exampe Whenever we insert a new tupe into Ses, make sure the beer mentioned is aso mentioned in Beers, and insert it (with a nu manufacturer) if not. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 385

29 Exampe (II) Ses(bar, beer, price) CREATE OR REPLACE TRIGGER BeerTrig AFTER INSERT ON Ses FOR EACH ROW WHEN(new.beer NOT IN (SELECT name FROM Beers)) BEGIN INSERT INTO Beers(name) VALUES(:new.beer); END;. Run Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 386

30 Options Can omit OR REPLACE. Effect is that it is an error if a trigger of this name exists. AFTER can be BEFORE. INSERT can be DELETE or UPDATE OF < attribute > ON. FOR EACH ROW can be omitted, with an important effect: the action is done once for the reation(s) consisting of a changes. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 387

31 Notes There are two specia variabes new and od, representing the new and od tupe in the change.»od makes no sense in an insert, and new makes no sense in a deete. Notice: in WHEN we use new and od without a coon, but in actions, a preceding coon is needed. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 388

32 Notes (II) The action is a PL/SQL statement.»simpest form: surround one or more SQL statements with BEGIN and END.»However, seect-from-where has a imited form. Dot and run cause the definition of the trigger to be stored in the database.»orace triggers are eements of the database, ike tabes or views. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 389

33 Exampe Maintain a ist of a the bars that raise their price for some beer by more than $1. Ses(bar, beer, price) Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 390

34 Exampe (II) CREATE TRIGGER PriceTrig AFTER UPDATE OF price ON Ses FOR EACH ROW WHEN(new.price > od.price ) BEGIN INSERT INTO RipoffBars VALUES(:new.bar); END;. run Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 391

35 SQL3 Triggers Some differences, incuding:» Position of FOR EACH ROW.» You can execute the action INSTEAD OF as we as before or after the event that triggered the action.» The Orace restriction about not modifying the reation of the trigger or other reations inked to it by constraints is not present in SQL3 (but Orace is rea; SQL3 is paper).» 4. The action in SQL3 is a ist of SQL3 statements, not a PL/SQL statement. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 392

36 SQL2 Assertions Database-schema constraint. Not present in Orace Checked whenever a mentioned reation changes. Syntax: CREATE ASSERTION <name> CHECK(<condition>); Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 393

37 Exampe No bar may charge an average of more than $5 for beer. Ses(bar, beer, price) CREATE ASSERTION NoRipoffBars CHECK(NOT EXISTS( SELECT bar FROM Ses GROUP BY bar HAVING 5.0 < AVG(price) ) ); Checked whenever Ses changes. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 394

38 Exampe There cannot be more bars than drinkers. Bars(name, addr, icense) Drinkers(name, addr, phone) CREATE ASSERTION FewBar CHECK( (SELECT COUNT(*) FROM Bars)!= (SELECT COUNT(*) FROM Drinkers) ); Checked whenever Bars or Drinkers changes. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 395

39 Chaenge Suppose we have our usua reations Beers(name, manf) Ses(bar, beer, price) and we want to maintain the foreign-key constraint that if you se a beer, its name must appear in Beers. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 396

40 Chaenge (II) Question: if we don't have foreign-key decarations avaiabe, how coud we arrange for this constraint to be maintained:»using attribute-based constraints?»using SQL2 assertions?»using Orace triggers? Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 397

41 Chaenge (III) Question: What if we aso want to make sure that each beer mentioned in Beers is sod at at east one bar? Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 398

42 Another Constraints Probem Reca our design of a DB for cities, counties and states, especiay how we decided that we needed a reationship cities-states to support the "weakness" of cities. A DB schema might have:» States(name, pop, capita)» Cities(name, pop, state)» Counties(name, pop, state)» CinC(cityName, citystate, countyname, countystate) Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 399

43 Another Constraints Probem (II) There is a funny form of redundancy, because we expect to be abe to find the state of a city by going to any county for that city and finding the state of that county. Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 400

44 Another Constraints Probem (III) Question: as written, is it possibe for a city to be in counties that are in two different states? How woud you enforce the requirement that that not happen? Question: if we are wiing to enforce the constraint, can you simpify the database schema? Shoud you? Fa, 2001, LRX #13 Outerjoins, Constraints, Triggers HUST,Wuhan,China 401