Instructor: Amol Deshpande

Transcription

1 Instructor: Amol Deshpande

2 } New topics to discuss More constructs in E/R modeling from E/R to schema some E/R models Ruby on Rails } Other things Grading of SQL: today or tomorrow Assignment/Project 2 released yesterday Make sure the basic setup is working many more configura7on issues compared to PostgreSQL Will start collec7ng in- class worksheets

3 } Model (E/R model) } Conver@ng from E/R to Rela@onal } Ruby on Rails

4 } a associates an set to itself } Need roles to dis@nguish course course_id title credits course_id prereq_id prereq

5 } An set without enough aqributes to have a primary key } E.g. Sec@on En@ty } S@ll need to be able to dis@nguish between weak en@@es Called discriminator aqributes : dashed underline course course_id title credits sec_course section sec_id semester year

6 } needed, and useful } Can replace with another en@ty set and three binary rela@onships project... instructor ID name salary proj_ guide student ID name tot_cred

7 Similar to object-oriented programming: allows inheritance etc. Disjoint vs Overlapping: No person can be both employee and student Partial vs Total There may be Persons who are neither employee or student person ID name address Different ways to convert to a Relational schema based on the above issues employee salary student tot_credits instructor rank secretary hours_per_week

8 } No allowed between } Suppose we want to record of a student by a guide on a project project instructor proj_ guide student eval_ for evaluation

9 } Nothing about actual data How is it stored? } No talk about the query languages How do we access the data? } Seman@c vs Syntac@c Data Models Remember: E/R Model is used for conceptual modeling Many conceptual models have the same proper@es } They are much more about represen@ng the knowledge than about database storage/querying

10 } Basic design principles Faithful Must make sense the requirements Models the requisite domain knowledge If not modeled, lost a[erwards Avoid redundancy for inconsistencies Go for simplicity } Typically an itera@ve process that goes back and forth

11 } sets vs aqributes Depends on the of the Consider telephone instructor ID name salary phone_number instructor ID name salary inst_phone phone phone_number location (a) (b)

12 } sets vs sets Consider takes section_reg registration student_reg section sec_id semester year student ID name tot_cred Figure 7.18 Replacement of takes by registration and two relationship sets

13 } sets vs aqributes Depends on the of the Consider telephone } En@ty sets vs Rela@onsihp sets Consider takes } N- ary vs binary rela@onships Possible to avoid n- ary rela@onships, but there are some cases where it is advantageous to use them } It is not an exact science!!

14 } Model diagram- based of domain knowledge, data etc Two key concepts: We also looked at: Keys Weak sets

15 } Model No standardized model (as far as I know) You will see different types of symbols/constructs Easy to reason about/understand/construct Not as easy to implement Came a[er the rela@onal model, so no real implementa@on was ever done Mainly used in the design phase

16 } Model (E/R model) } Conver@ng from E/R to Rela@onal } Ruby on Rails

17 } Convert sets into a rela@onal schema with the same set of aqributes Customer Customer_Schema(cname, ccity, cstreet) cname ccity cstreet bname bcity assets Branch Branch_Schema(bname, bcity, assets)

18 } Convert sets also into a rela@onal schema } Remember: A rela@onship is completely described by primary keys of associate en@@es and its own aqributes acct-no balance Account access-date Depositor Account_Schema(acct-no, balance) Depositor_Schema(cname, acct-no, access-date) Customer cname ccity cstreet Customer_Schema(cname, ccity, cstreet) Well Not quite. We can do better. It depends on the relationship cardinality

19 } Say One- to- Many from Customer to Account à Many accounts per customer acct-no balance Account access-date Account_Schema(acct-no, balance, cname, access-date) Depositor Customer cname ccity cstreet Customer_Schema(cname, ccity, cstreet) Exactly same information, fewer tables

20 Entity Sets E/R E 1 Relational Schema E = (a 1,, a n ) a 1 a n Relationship Sets E 1 R E 2 R = (a 1, b 1, c 1,, c n ) a 1 a n b 1 b m a 1 : E 1 s key c 1 c k b 1 : E 2 s key c 1,, c k : attributes of R Not the whole story for Relationship Sets

21 Relationship Cardinality Relational Schema E 1 R E 2 a 1 a n c1 ck b 1 b m n:m R E 1 = (a 1,, a n ) E 2 = (b 1,, b m ) R = (a 1, b 1, c 1,, c n ) n:1 E 1 = (a 1,, a n, b 1, c 1,, c n ) R E 2 = (b 1,, b m ) 1:n E 1 = (a 1,, a n ) R E 2 = (b 1,, b m,, a 1, c 1,, c n ) 1:1 R Treat as n:1 or 1:n

22 acct_no balance bname bcity assets Account Acct-Branch Branch Depositor Loan-Branch Customer Borrower Loan cname ccity cstreet lno amt Q. How many tables does this get translated into? A. 6 (account, branch, customer, loan, depositor, borrower)

23 Weak Entity Sets E 1 E/R IR E 2 Relational Schema E 1 = (a 1,, a n ) E 2 = (a 1, b 1,, b m ) a 1 a n b 1 b m

24 E/R Relational Schema Multivalued Attributes Employee Emp Emp-Phones = (ssn, name) = (ssn, phone) ssn name phone ssn name ssn phone 001 Smith Emp Emp-Phones

25 E/R Relational Schema Subclasses Method 1: a 1 E a n E = (a 1,, a n ) E 1 = (a 1, b 1,, b m ) E 2 = (a 1, c 1,, c k ) ISA Method 2: E 1 E 2 E 1 = (a 1,, a n, b 1,, b m ) E 2 = (a 1,, a n, c 1,, c k ) b b 1 m c 1 c k

26 } Subclasses example: } Method 1: } Account = (acct_no, balance) } SAccount = (acct_no, interest) } CAccount = (acct_no, overdra[) } Method 2: } SAccount = (acct_no, balance, interest) } CAccount = (acct_no, balance, overdra[) Q: When is method 2 not possible? A: When subclassing is partial

27 } Model (E/R model) } Conver@ng from E/R to Rela@onal } Ruby on Rails

28 } Web framework wriqen in Ruby } Uses a Model- View- Controller paqern } Paradigms: Conven@on- over- configura@on Don t repeat yourself Ac@ve record paqern

29 } Models (stored in app/models) Map to database tables E.g., Instructor model maps to instructors table in the database Can keep track of between different models (e.g., foreign keys, etc). } Controllers (stored in app/controllers) Responds to external requests from the web server Manipulates the models and passes informa@on to views } Views (stored in app/views) ERB files that control what is shown to the user Get translated into HTML } Config/routes.rb Dictates what controllers to call in response to different requests

30 } Basic skeleton already created for you } You have to figure out what more en@@es to add, and set up the controllers/views/models appropriately Probably don t need to add anything to config/routes.rb } To get started: Create the student lis@ng and course lis@ng pages Understand how to set up associa@ons between different models (see appropriate Rails guide)

31 } Many tutorials on the web } Rails Guides are very good } Rails for Zombies great place to get started