Transcript Slides
Entity-Relationship Model Database Modeling • The process of designing a database begins with: • an analysis of what information the database must hold, and • the relationships among components of that information. • The structure of the database, called the database schema is specified in a suitable notation for expressing designs. • We will use the Entity-Relationship (E/R) model, which is graphical in the nature. Ideas E/R design Abstract design Relational schema A mechanical process Relational DBMS Concrete design Elements of the E/R Model • Entities are like the objects in OO programming. Entity sets are like the classes. • • • However, E/R is a static model, so there are no methods associated with the entities. Attributes are similar to the attributes in OO programming. • Movies Title However, they are atomic, i.e. only numbers and strings are allowed. Relationships are connections among two or more entity sets. Stars-in Example year title Movies name Stars-In address Stars name length filmType Owns Studios address Visualizing E/R Relationships • It is often helpful (for us) to represent E/R relationships by a table or relation, each row representing a list (it can be pair) of entities participating in the relationship. For instance the Stars-in relationship could be visualized as a table: Movies Basic Instinct Total Recall Total Recall | | | | Stars Sharon Stone. Arnold Schwarzeneger. Sharon Stone. • This table is called the relationship set for the relationship. • The members of the relationship set are the rows of the table. Multiplicity of Relationships • A relationship R is many-one from one entity set E to another entity set F, if any member of E can be associated to at most one member of F. • We also equivalently say that R is one-many from F to E. Any member of F can be associated with many members of E. Movies • Owns Studios Arrows can be used to indicate the multiplicity of a relationship in an E/R diagram. • • An arrow means “at most one.” It does not guarantee the existence of an entity of the set pointed to. So, if a relationship is many-one from entity set E to F, we place an arrow entering F. Multiplicity (Cont.) • If a relationship R is both many-one from E to F and many-one from F to E, then we say that R is one-one • represented by arrows pointing to both E and F. Studios • Runs Presidents If a relationship R is neither many-one from E to F or from F to E, then we say R is many-many. Movies Stars-In Stars Multiway Relationships • The E/R model makes it convenient to define relationships involving more than two entity sets. • A multiway relationship in an E/R diagram is represented by lines from the relationship diamond to each of the involved entity sets. • Motivating Example. Take the previous organization: Movies Stars-In Owns Studios What’s the problem with this design? Stars Relating studios, movies and stars • Suppose that we want to know how studios are related to the stars. I.e. Which stars a studio is (or was) paying for a specific movie. • As long as a movie is produced by a single studio the previous organization works very well. • Why? Because we can relate a studio s through a specific movie m with the stars sti forming valid triples: (s, m, st1), (s, m, st2),…, (s, m, stk) • Suppose now that movies can be produced (and owned) by not just a single studio but, by some studios. (The line connecting studios is without arrow in such a case) • Now if we try to create the above triples they can be not valid. Why the triples can be not valid? • Suppose a studio s1 is paying a star st1 for the movie m and a studio s2 is paying st2 for the same movie m. • Then we will have the following triples: (s1, m, st1) (s1, m, st2) (s2, m, st1) (s2, m, st2) • The second and the third triples are not valid. • If we consider the collection of all the valid triples, it is nothing else but a three way relationship between Studios, Movies and Stars Solution: Three-way relationship Stars Contracts Movies Studios Formally: • The relationship Contracts involves a studio, a star, and a movie. • This relationship says that a studio has contracted a particular star in a particular movie. • In multiway relationships, an arrow pointing to an entity set E means: If we select one entity from each of the other entity sets in the relationship, these entities are related to at most one entity in E. Roles in a relationship • An entity set can appear two or more times in a relationship. • If so, we draw as many lines from the relationship to the entity set as the entity set appears in the relationship. • Each line to the entity set represents a different role. Original Movies Sequel-of Sequel • We assume that a movie may have many sequels, but for each sequel there is only one original movie. Another Example Relationship Set Husband Bob Joe … Married husband wife Drinkers Wife Ann Sue … Another Example Relationship Set Buddies 1 2 Drinkers Buddy1 Bob Joe Ann Joe … Buddy2 Ann Sue Bob Moe … Attributes on Relationships • Sometimes, it is convenient to associate attributes with a relationship, rather than with any one of the entity sets that the relationship connects. • Consider the 3-way Contracts relationships between a star and a studio for a movie. • We might wish to record the salary associated with this contract. • We cannot associate it with the star; a star might get different salaries for different movies. • It does not make sense to associate the salary with a studio or with a movie. • However, it is appropriate to associate a salary with the (star, movie, studio) triple in the relationship set for the Contracts relationship. A relationship with an attribute title year name address salary Movies Stars Contracts length filmType Studios name address “Bars-Beer-Drinkers” (BBD) Example • Bars sell some beers. • Drinkers like some beers. • Drinkers frequent some bars. • What would be the E/R diagram? “Bars-Beer-Drinkers” (BBD) Example name addr name Bars Beers Sells license Note: license = beer, full, none Frequents name Why we need it? Drinkers manf Likes Bars sell some beers. Drinkers like some beers. Drinkers frequent some bars. addr BBD relation sets • For the relationship Sells, we might have a relationship set like: Bar Joe’s Bar Joe’s Bar Sue’s Bar Sue’s Bar Sue’s Bar Beer Bud Miller Bud Pete’s Ale Bud Lite BBD Multiway Relationship • Suppose that drinkers will only drink certain beers at certain bars. – Our three binary relationships Likes, Sells, and Frequents do not allow us to make this distinction. – But a 3-way relationship would. name license addr name Bars Beers Preferences Drinkers name addr manf A Typical Relationship Set Bar Joe’s Bar Sue’s Bar Sue’s Bar Joe’s Bar Joe’s Bar Joe’s Bar Sue’s Bar Drinker Ann Ann Ann Bob Bob Cal Cal Beer Miller Bud Pete’s Ale Bud Miller Miller Bud Lite Multiple Relationships Between Two Entity Sets Drinkers Likes Favorite Beers “Exactly one” Multiplicity Manfs Bestseller Beers Some beers are not the best-seller of any manufacturer, so a rounded arrow to Manfs would be inappropriate. But a manufacturer has to have a bestseller (we assume they are beer manufacturers).