Instructor:  Amol  Deshpande                                        amol@cs.umd.edu  

}  New  topics  to  discuss  ◦  More  constructs  in  E/R  modeling  ◦  Conver@ng  from  E/R  to  rela@onal  schema  ◦  Crea@ng  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  

}  En@ty-­‐rela@onship  Model  (E/R  model)    }  Conver@ng  from  E/R  to  Rela@onal  

}  Ruby  on  Rails  

}  Some@mes  a  rela@onship  associates  an  en@ty  set  to  itself  

}  Need  “roles”  to  dis@nguish  

coursecourse_idtitlecredits

course_id

prereq_id prereq

}  An  en@ty  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    

coursecourse_idtitlecredits

sectionsec_idsemesteryear

sec_course

}  Some@mes  needed,  and  useful  }  Can  replace  with  another  en@ty  set  and  three  binary  rela@onships     7.5 Entity-Relationship Diagrams 279

instructor

IDnamesalary

student

IDnametot_cred

. . .project

proj_guide

Figure 7.13 E-R diagram with a ternary relationship.

We can specify some types of many-to-one relationships in the case of non-binary relationship sets. Suppose a student can have at most one instructor asa guide on a project. This constraint can be specified by an arrow pointing toinstructor on the edge from proj guide.

We permit at most one arrow out of a relationship set, since an E-R diagramwith two or more arrows out of a nonbinary relationship set can be interpreted intwo ways. Suppose there is a relationship set R between entity sets A1, A2, . . . , An,and the only arrows are on the edges to entity sets Ai+1, Ai+2, . . . , An. Then, thetwo possible interpretations are:

1. A particular combination of entities from A1, A2, . . . , Ai can be associatedwith at most one combination of entities from Ai+1, Ai+2, . . . , An. Thus, theprimary key for the relationship R can be constructed by the union of theprimary keys of A1, A2, . . . , Ai .

2. For each entity set Ak , i < k ≤ n, each combination of the entities from theother entity sets can be associated with at most one entity from Ak . Each set{A1, A2, . . . , Ak−1, Ak+1, . . . , An}, for i < k ≤ n, then forms a candidate key.

Each of these interpretations has been used in different books and systems. Toavoid confusion, we permit only one arrow out of a relationship set, in whichcase the two interpretations are equivalent. In Chapter 8 (Section 8.4), we studyfunctional dependencies, which allow either of these interpretations to be specifiedin an unambiguous manner.

7.5.6 Weak Entity Sets

Consider a section entity, which is uniquely identified by a course identifier,semester, year, and section identifier. Clearly, section entities are related to courseentities. Suppose we create a relationship set sec course between entity sets sectionand course.

Now, observe that the information in sec course is redundant, since sectionalready has an attribute course id, which identifies the course with which thesection is related. One option to deal with this redundancy is to get rid of the

personIDnameaddress

student

instructorrank

secretaryhours_per_week

employeesalary tot_credits

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

Different ways to convert to a Relational schema based on the above issues

}  No  rela@onships  allowed  between  rela@onships  }  Suppose  we  want  to  record  evalua@ons  of  a  student  by  a  guide  on  a  project  

evaluation

proj_ guideinstructor student

eval_ for

project

}  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  

}  Basic  design  principles  ◦  Faithful  �  Must  make  sense  ◦  Sa@sfies  the  applica@on  requirements  ◦  Models  the  requisite  domain  knowledge  �  If  not  modeled,  lost  a[erwards  ◦  Avoid  redundancy  �  Poten@al  for  inconsistencies  ◦  Go  for  simplicity  

}  Typically  an  itera@ve  process  that  goes  back  and  forth      

}  En@ty  sets  vs  aQributes  ◦  Depends  on  the  seman@cs  of  the  applica@on  ◦  Consider  telephone  

290 Chapter 7 Database Design and the E-R Model

added to the instructor relation when the schema for inst dept is merged intoinstructor.

7.7 Entity-Relationship Design Issues

The notions of an entity set and a relationship set are not precise, and it is possibleto define a set of entities and the relationships among them in a number ofdifferent ways. In this section, we examine basic issues in the design of an E-Rdatabase schema. Section 7.10 covers the design process in further detail.

7.7.1 Use of Entity Sets versus Attributes

Consider the entity set instructor with the additional attribute phone number (Fig-ure 7.17a.) It can easily be argued that a phone is an entity in its own right withattributes phone number and location; the location may be the office or home wherethe phone is located, with mobile (cell) phones perhaps represented by the value“mobile.” If we take this point of view, we do not add the attribute phone numberto the instructor. Rather, we create:

• A phone entity set with attributes phone number and location.

• A relationship set inst phone, denoting the association between instructorsand the phones that they have.

This alternative is shown in Figure 7.17b.What, then, is the main difference between these two definitions of an instruc-

tor? Treating a phone as an attribute phone number implies that instructors haveprecisely one phone number each. Treating a phone as an entity phone permitsinstructors to have several phone numbers (including zero) associated with them.However, we could instead easily define phone number as a multivalued attributeto allow multiple phones per instructor.

The main difference then is that treating a phone as an entity better modelsa situation where one may want to keep extra information about a phone, suchas its location, or its type (mobile, IP phone, or plain old phone), or all who share

instructor

IDnamesalary

phonephone_numberlocation

instructor

IDnamesalaryphone_number

(a) (b)

inst_phone

Figure 7.17 Alternatives for adding phone to the instructor entity set.

}  En@ty  sets  vs  Rela@onsihp  sets  ◦  Consider  takes  

292 Chapter 7 Database Design and the E-R Model

registration.........

sectionsec_idsemesteryear

studentIDnametot_cred

section_reg student_reg

Figure 7.18 Replacement of takes by registration and two relationship sets

Both the approach of Figure 7.15 and that of Figure 7.18 accurately representthe university’s information, but the use of takes is more compact and probablypreferable. However, if the registrar’s office associates other information with acourse-registration record, it might be best to make it an entity in its own right.

One possible guideline in determining whether to use an entity set or arelationship set is to designate a relationship set to describe an action that occursbetween entities. This approach can also be useful in deciding whether certainattributes may be more appropriately expressed as relationships.

7.7.3 Binary versus n-ary Relationship Sets

Relationships in databases are often binary. Some relationships that appear to benonbinary could actually be better represented by several binary relationships.For instance, one could create a ternary relationship parent, relating a child tohis/her mother and father. However, such a relationship could also be representedby two binary relationships, mother and father, relating a child to his/her motherand father separately. Using the two relationships mother and father provides us arecord of a child’s mother, even if we are not aware of the father’s identity; a nullvalue would be required if the ternary relationship parent is used. Using binaryrelationship sets is preferable in this case.

In fact, it is always possible to replace a nonbinary (n-ary, for n > 2) relation-ship set by a number of distinct binary relationship sets. For simplicity, considerthe abstract ternary (n = 3) relationship set R, relating entity sets A, B, and C . Wereplace the relationship set R by an entity set E , and create three relationship setsas shown in Figure 7.19:

• RA, relating E and A.

• RB , relating E and B.

• RC , relating E and C .

}  En@ty  sets  vs  aQributes  ◦  Depends  on  the  seman@cs  of  the  applica@on  ◦  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  !!  

}  En@ty-­‐rela@onship  Model  ◦  Intui@ve  diagram-­‐based  representa@on  of  domain  knowledge,  data  proper@es  etc…  ◦  Two  key  concepts:  �  En@@es  �  Rela@onships  ◦  We  also  looked  at:  �  Rela@onship  cardinali@es  �  Keys  �  Weak  en@ty  sets  �  …  

}  En@ty-­‐rela@onship  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  

}  En@ty-­‐rela@onship  Model  (E/R  model)    }  Conver@ng  from  E/R  to  Rela@onal  

}  Ruby  on  Rails  

}  Convert  en@ty  sets  into  a  rela@onal  schema  with  the  same  set  of  aQributes  

Customer

cname ccity cstreet

Customer_Schema(cname, ccity, cstreet)

Branch

bname bcity assets Branch_Schema(bname, bcity, assets)

}  Convert  rela@onship  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  

Depositor_Schema(cname, acct-no, access-date)

Account

Customer

Depositor

acct-no balance

cname ccity cstreet

access-date

Well… Not quite. We can do better. It depends on the relationship cardinality

Customer_Schema(cname, ccity, cstreet)

Account_Schema(acct-no, balance)

}  Say  One-­‐to-­‐Many  Rela@onship  from  Customer  to  Account                à  Many  accounts  per  customer  

Account

Customer

Depositor

acct-no balance

cname ccity cstreet

access-date

Customer_Schema(cname, ccity, cstreet)

Account_Schema(acct-no, balance, cname, access-date)

Exactly same information, fewer tables

E/R Relational Schema Entity Sets

E = (a1, …, an)

Relationship Sets R = (a1, b1, c1, …, cn)

a1: E1’s key b1: E2’s key

c1, …, ck: attributes of R

Not the whole story for Relationship Sets …

E1

… a1 an

E1

… …

…

R E2

a1 an

c1 ck

b1 bm

Relationship Cardinality Relational Schema

n:m E1 = (a1, …, an) E2 = (b1, …, bm) R = (a1, b1, c1, …, cn)

n:1 E1 = (a1, …, an, b1, c1, …, cn) E2 = (b1, …, bm)

1:n E1 = (a1, …, an) E2 = (b1, …, bm,, a1, c1, …, cn)

1:1 Treat as n:1 or 1:n

R

R

R

R

E1

… …

…

R E2

a1 an

c1 ck

b1 bm

Acct-Branch Account Branch

Borrower Customer Loan

Depositor Loan-Branch

Q. How many tables does this get translated into? A. 6 (account, branch, customer, loan, depositor, borrower)

acct_no balance bname bcity assets

cname ccity cstreet lno amt

E/R Relational Schema Weak Entity Sets

E1 = (a1, …, an) E2 = (a1, b1, …, bm)

E1

… …

IR E2

a1 an b1 bm

E/R Relational Schema Multivalued Attributes

Emp = (ssn, name) Emp-Phones = (ssn, phone)

Emp

ssn name 001 …

Smith …

Emp-Phones

Employee

ssn name phone ssn phone 001 001 …

4-1234 4-5678 …

E/R Relational Schema Subclasses

Method 1: E = (a1, …, an) E1 = (a1, b1, …, bm) E2 = (a1, c1, …, ck) Method 2: E1 = (a1, …, an, b1, …, bm) E2 = (a1, …, an, c1, …, ck)

E

E2

ISA

E1

… … b1 bm c1 ck

… a1 an

}  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

}  En@ty-­‐rela@onship  Model  (E/R  model)    }  Conver@ng  from  E/R  to  Rela@onal  

}  Ruby  on  Rails  

}  Web  applica@on  framework  wriQen  in  Ruby  }  Uses  a  Model-­‐View-­‐Controller  paQern  }  Paradigms:  ◦  Conven@on-­‐over-­‐configura@on  ◦  Don’t  repeat  yourself  ◦  Ac@ve  record  paQern  

}  Models  (stored  in  app/models)  ◦  Map  to  database  tables  ◦  E.g.,  Instructor  model  maps  to  instructors  table  in  the  database  ◦  Can  keep  track  of  associa@ons  between  different  models  (e.g.,  rela@onships,  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  

}  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)  

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