ODL Sub classes F ollo w name of sub class b y colon and - PDF document

ODL Sub classes F ollo w name of sub class b y colon and its sup erclass. Example: Ales are Beers with a Color class Ales:Beers { attribute string color; } Ob jects of the class acquire all the Ales � attributes and relationships of the class. Beers While E/R en tities can ha v e manifestations in � a class and sub class, in ODL w e assume eac h ob ject is a mem b er of exactly one class. 1

Keys in ODL Indicate with follo wing the class name, and key(s) a list of attributes forming the k ey . Sev eral lists ma y b e used to indicate sev eral � alternativ e k eys. P aren theses group mem b ers of a k ey , and also � group to the declared k eys. key Th us, = \one (key( a ; a ; : : : ; a )) � 1 2 n k ey consisting of all attributes." n = \eac h is a k ey b y a ; a ; : : : ; a a (key ) 1 2 n i itself." Example class Beers (key name) { attribute string name ... : Keys are optional in ODL. The R ememb er � \ob ject ID" su�ces to distinguish ob jects that ha v e the same v alues in their elemen ts. 2

Example: Multiple Multiattribut e Keys class Courses (key (dept, number), (room, hours)) { ... 3

T ranslating ODL to Relations 1. Classes without relationships: lik e en tit y set, but sev eral new problems arise. 2. Classes with relationships: a) T reat the relationship separately , as in E/R. b) A ttac h a man y-one relationship to the relation for the \man y ." 4

ODL Class Without Relationships Problem: ODL allo ws attribute t yp es built � from structures and collecti on t yp es. Structure: Mak e one attribute for eac h �eld. � Set: mak e one tuple for eac h mem b er of the � set. ✦ More than one set attribute? Mak e tuples for all com binations. Problem: ODL class ma y ha v e no k ey , but w e � should ha v e one in the relation to represen t \OID." 5

Example class Drinkers (key name) { attribute string name; attribute Struct Addr {string street, string city, int zip} address; attribute Set<string> phone; } name street cit y zip phone n s c z p 1 1 1 1 1 n s c z p 1 1 1 1 2 Surprise: the k ey for the class (name) is not � the k ey for the relation (name, phone). ✦ in the class determines a unique name ob ject, including a of phones. set ✦ in the relation do es not determine a name unique tuple. ✦ Since tuples are not iden tical to ob jects, there is no inconsistency! BCNF violati on: separate out name-phone. � 6

ODL Relationships If the relationship is man y-one from to , A B � put k ey of attributes in the relation for class B A . If relationship is man y-man y , w e'll ha v e to � duplicate A -tuples as in ODL with set-v alued attributes. ✦ W ouldn't y ou really rather create a separate relation for a man y-man y- relationship? ✦ Y ou'll wind up separating it an yw a y , during BCNF decomp osition. 7

Example class Drinkers (key name) { attribute string name; attribute string addr; relationship Set<Beers> likes inverse Beers::fans; relationship Beers favorite inverse Beers::realFans; relationship Drinkers husband inverse wife; relationship Drinkers wife inverse husband; relationship Set<Drinkers> buddies inverse buddies; } Drink ers(name, addr, b eerName , fa vBeer, wife, budd y) 8

Decomp ose in to 4NF FD's: name ! addr fa vBeer wife � MVD's name ! b eerName, name ! buddy � ! ! Resulting decomp osition: � Drinkers(name , addr, favBeer, wife) DrBeer(name , beer ) DrBuddy(name , buddy) 9

OQL Motiv atio n: Relational languages su�er from imp e danc e � when w e try to connect them to mismatch con v en tional languages lik e C or C++. ✦ The data mo dels of C and SQL are radically di�eren t, e.g. C do es not ha v e relations, sets, or bags as primitiv e t yp es; C is tuple-at-a-time, SQL is relation-at-a- time. OQL is an attempt b y the OO comm unit y � to extend languages lik e C++ with SQL-lik e, relation-at-a-time dictions. 10

OQL T yp es Basic t yp es: strings, in ts, reals, etc., plus class � names. T yp e constructors: � ✦ for structures. Struct ✦ Collecti on t yp es: set, bag, list, arra y . Lik e ODL, but no limit on the n um b er of � times w e can apply a t yp e constructor. Set(Struct()) and Bag(Struct()) pla y sp ecial � roles akin to relations. 11

OQL Uses ODL as its Sc hema-De�nition P ortion F or ev ery class w e can declare an = extent � name for the curren t set of ob jects of the class. ✦ Remem b er to refer to the exten t, not the class name, in queries. 12

class Bar (extent Bars) { attribute string name; attribute string addr; relationship Set<Sell> beersSold inverse Sell::bar; } class Beer (extent Beers) { attribute string name; attribute string manf; relationship Set<Sell> soldBy inverse Sell::beer; } class Sell (extent Sells) { attribute float price; relationship Bar bar inverse Bar::beersSold; relationship Beer beer inverse Beer::soldBy; } 13

P ath Expressions Let b e an ob ject of class . x C If is an attribute of , then = the v alue a C x:a � of in the ob ject. a x If is a relationship of , then = the r C x:r � v alue to whic h is connected b y . x r ✦ Could b e an ob ject or a collect i on of ob jects, dep ending on the t yp e of . r If is a metho d of , then x:m ( � � ) is the m C � � result of applying to x . m 14

Examples Let b e a v ariable whose t yp e is Sell . s = the price in the ob ject s . s.price � = the address of the bar s.bar.addr � men tioned in s . ✦ Note: cascade of dots OK b ecause s.bar is an ct , not a collecti on. obje Example of Illegal Use of Dot b.beersSold.price , where is a ob ject. b Bar Wh y illegal ? Because is a of b.beersSold set � ob jects, not a single ob ject. 15

OQL Select-F rom-Where < list of v alues > SELECT < list of collecti ons and FROM t ypical mem b ers > < condition > WHERE Collecti ons in can b e: FROM � 1. Exten ts. 2. Expressions that ev aluate to a collecti on. F ollo wing a collecti on is a name for a t ypical � mem b er, optionally preceded b y AS . Example Get the men u at Jo e's. SELECT s.beer.name, s.price FROM Sells s WHERE s.bar.name = "Joe's Bar" Notice double-quoted strings in OQL. � 16

Example Another w a y to get Jo e's men u, this time fo cusing on the ob jects. Bar SELECT s.beer.name, s.price FROM Bars b, b.beersSold s WHERE b.name = "Joe's Bar" Notice that the t ypical ob ject in the �rst b � collecti on of is used to help de�ne the FROM second collecti on. T ypical Usage If is an ob ject, y ou can extend the path x � expression, lik e or in s.beer.name . s s.beer If is a collect i on, y ou use it in the list, x FROM � lik e ab o v e, if y ou w an t to access b.beersSold attributes of x . 17

T ailoring the T yp e of the Result Default: bag of structs, �eld names tak en from � the ends of path names in clause. SELECT Example SELECT s.beer.name, s.price FROM Bars b, b.beersSold s WHERE b.name = "Joe's Bar" has result t yp e: Bag(Struct( name: string, price: real )) 18

Rename Fields Pre�x the path with the desired name and a colon. Example SELECT beer: s.beer.name, s.price FROM Bars b, b.beersSold s WHERE b.name = "Joe's Bar" has t yp e: Bag(Struct( beer: string, price: real )) 19

Change the Collectio n T yp e Use to get a set of structs. SELECT DISTINCT � Example SELECT DISTINCT s.beer.name, s.price FROM Bars b, b.beersSold s WHERE b.name = "Joe's Bar" Use clause to get a list of structs. ORDER BY � Example joeMenu = SELECT s.beer.name, s.price FROM Bars b, b.beersSold s WHERE b.name = "Joe's Bar" ORDER BY s.price ASC = ascending (default); = descending. ASC DESC � W e can extract from a list as if it w ere an � arra y , e.g. cheapest = joeMenu[1].name; 20