Likesèdrinker, beerè. Sellsèbar, beer, priceè. Frequentsèdrinker, barè

Transcription

1 Modication to Views Via Triggers Oracle allows us to ëintercept" a modication to a view through an instead-of trigger Example Likesèdrinker, beerè Sellsèbar, beer, priceè Frequentsèdrinker, barè CREATE VIEW Synergy AS SELECT Likesdrinker, Likesbeer, Sellsbar FROM Likes, Sells, Frequents WHERE Likesdrinker = Frequentsdrinker AND Likesbeer = Sellsbeer AND Sellsbar = Frequentsbar; 1

2 CREATE TRIGGER ViewTrig INSTEAD OF INSERT ON Synergy FOR EACH ROW INSERT INTO Likes VALUESè :newdrinker, :newbeerè; INSERT INTO Sellsèbar, beerè VALUESè:newbar, :newbeerè; INSERT INTO Frequents VALUESè :newdrinker, :newbarè; run 2

3 SQL Triggers Read in text Some dierences, including: 1 The Oracle restriction about not modifying the relation of the trigger or other relations linked to it by constraints is not present in SQL èbut Oracle is real; SQL is paperè 2 The action in SQL is a list of èrestrictedè SQL statements, not a PLèSQL statement 3

4 PLèSQL Oracle's version of PSM èpersistent, Stored Modulesè Use via sqlplus A compromise between completely procedural programming and SQL's very high-level, but limited statements Allows local variables, loops, procedures, examination of relations one tuple at a time Rough form: DECLARE declarations executable statements run; DECLARE portion is optional Dot and run èor a slash in place of run;è are needed to end the statement and execute it 4

5 Simplest Form: Sequence of Modications Likesèdrinker, beerè INSERT INTO Likes VALUESè'Sally', 'Bud'è; DELETE FROM Likes WHERE drinker = 'Fred' AND beer = 'Miller'; run; 5

6 Procedures Stored database objects that use a PLèSQL statement in their body Procedure Declarations CREATE OR REPLACE PROCEDURE énameéèéarglistéè AS édeclarationsé éplèsql statementsé run; 6

7 Argument list has name-mode-type triples Mode: IN, OUT, or IN OUT for readonly, write-only, readèwrite, respectively Types: standard SQL + generic types like NUMBER = any integer or real type Since types in procedures must match their types in the DB schema, you should generally use an expression of the form relationattributeètype to capture the type correctly 7

8 Example A procedure to take a beer and price and add it to Joe's menu Sellsèbar, beer, priceè CREATE PROCEDURE joemenuè b IN SellsbeerèTYPE, p IN SellspriceèTYPE èas INSERT INTO Sells VALUESè'Joe''s Bar', b, pè; run; Note ërun" only stores the procedure; it doesn't execute the procedure 8

9 Invoking Procedures A procedure call may appear in the body of a PLèSQL statement Example: joemenuè'bud', 250è; joemenuè'moosedrool', 500è; run; 9

10 Assignment Assign expressions to declared variables with := Branches IF éconditioné THEN éstatementèsèé ELSE éstatementèsèé END IF; But in nests, use ELSIF in place of ELSE IF Loops LOOP EXIT WHEN éconditioné END LOOP; 10

11 Queries in PLèSQL 1 Single-row selects allow retrieval into a variable of the result of a query that is guaranteed to produce one tuple 2 Cursors allow the retrieval of many tuples, with the cursor and a loop used to process each in turn 11

12 Single-Row Select Select-from-where in PLèSQL must have an INTO clause listing variables into which a tuple can be placed It is an error if the select-from-where returns more than one tuple; you should have used a cursor Example Find the price Joe charges for Bud èand drop it on the oorè Sellsèbar, beer, priceè DECLARE p SellspriceèTYPE; SELECT price INTO p FROM Sells WHERE bar = 'Joe''s Bar' AND beer = 'Bud'; run 12

13 Cursors Declare by: CURSOR énameé IS select-from-where statement Cursor gets each tuple from the relation produced by the select-from-where, in turn, using a fetch statement in a loop Fetch statement: FETCH écursor nameé INTO variable list; Break the loop by a statement of the form: EXIT WHEN écursor nameéènotfound; True when there are no more tuples to get Open and close the cursor with OPEN and CLOSE 13

14 Example A procedure that examines the menu for Joe's Bar and raises by $100 all prices that are less than $300 Sellsèbar, beer, priceè This simple price-change algorithm can be implemented by a single UPDATE statement, but more complicated price changes could not 14

15 CREATE PROCEDURE joegougeèè AS thebeer SellsbeerèTYPE; theprice SellspriceèTYPE; CURSOR c IS SELECT beer, price FROM Sells WHERE bar = 'Joe''s bar'; OPEN c; LOOP FETCH c INTO thebeer, theprice; EXIT WHEN cènotfound; IF theprice é 300 THEN UPDATE Sells SET price = theprice WHERE bar = 'Joe''s Bar' AND beer = thebeer; END IF; END LOOP; CLOSE c; run 15

16 Row Types Anything èeg, cursors, table namesè that has a tuple type can have its type captured with èrowtype We can create temporary variables that have tuple types and access their components with dot Handy when we deal with tuples with many attributes 16

17 Example The same procedure with a tuple variable bp CREATE PROCEDURE joegougeèè AS CURSOR c IS SELECT beer, price FROM Sells WHERE bar = 'Joe''s bar'; bp cèrowtype; OPEN c; LOOP FETCH c INTO bp; EXIT WHEN cènotfound; IF bpprice é 300 THEN UDPATE Sells SET price = bpprice WHERE bar = 'Joe''s Bar' AND beer = bpbeer; END IF; END LOOP; CLOSE c; run 17