Database Conceptual and Logical Design

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Database Conceptual and Logical Design

• Zachary G. Ives/Grigoris Karvounarakis
• University of Pennsylvania
• CIS 550 Database Information Systems
• October 3, 2007

Some slide content courtesy of Susan Davidson
Raghu Ramakrishnan
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Now How Do We Get the Database in the First
Place?

• Database design theory!
• Neat outcome we can actually prove that we have
  optimal design, in a manner of speaking
• But first we need to understand how to visualize
  in pretty pictures

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Databases AnonymousA 6-Step Program

• Requirements Analysis what data, apps, critical
  operations
• Conceptual DB Design high-level description of
  data and constraints typically using ER model
• Logical DB Design conversion into a schema
• Schema Refinement normalization (eliminating
  redundancy)
• Physical DB Design consider workloads, indexes
  and clustering of data
• Application/Security Design

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Entity-Relationship Diagram(based on our running
example)
Underlined attributes are keys
PROFESSORS
fid
name
relationship set
Teaches
entity set
semester
Takes
STUDENTS
COURSES
serno
subj
cid
sid
name
exp-grade
attributes (recall these have domains)
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Conceptual Design Process

• What are the entities being represented?
• What are the relationships?
• What info (attributes) do we store about each?
• What keys integrity constraints do we have?

STUDENTS
Takes
name
exp-grade
sid
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Translating Entity Sets toLogical Schemas SQL
DDL
Fairly straightforward to generate a schema
CREATE TABLE STUDENTS (sid INTEGER,
name VARCHAR(15) PRIMARY KEY (sid) )
CREATE TABLE COURSES (serno INTEGER,
subj VARCHAR(30), cid CHAR(15),
PRIMARY KEY (serno) )
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Translating Relationship Sets

• Generate schema with attributes consisting of
• Key(s) of each associated entity (foreign keys)
• Descriptive attributes

CREATE TABLE Takes (sid INTEGER, serno
INTEGER, exp-grade CHAR(1), PRIMARY
KEY (?), FOREIGN KEY (serno) REFERENCES
COURSES, FOREIGN KEY (sid) REFERENCES
STUDENTS)
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OK, But What about Connectivityin the E-R
Diagram?

• Attributes can only be connected to entities or
  relationships
• Entities can only be connected via relationships
• As for the edges, lets consider kinds of
  relationships and integrity constraints

Teaches
PROFESSORS
COURSES
(warning the book has a slightly different
notation here!)
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Logical Schema Design

• Roughly speaking, each entity set or relationship
  set becomes a table (not always be the case see
  Monday)
• Attributes associated with each entity set or
  relationship set become attributes of the
  relation the key is also copied (ditto with
  foreign keys in a relationship set)

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Binary Relationships Participation

• Binary relationships can be classified as 11,
  1Many, or ManyMany, as in

1n
mn
11
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1Many (1n) Relationships

• Placing an arrow in the many ? one direction,
  i.e. towards the entity thats refd via a
  foreign key
• Suppose profs teach multiple courses, but may not
  have taught yet
• Suppose profs must teach to be on the roster

Teaches
PROFESSORS
COURSES
Partial participation (0 or more)
Teaches
PROFESSORS
COURSES
Total participation (1 or more)
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Many-to-Many Relationships

• Many-to-many relationships have no arrows on
  edges
• The relationship set relation has a key that
  includes the foreign keys, plus any other
  attributes specified as key

Takes
COURSES
STUDENTS
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Examples

• Suppose courses must be taught to be on the
  roster
• Suppose students must have enrolled in at least
  one course

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Representing 1n Relationships in Tables
CREATE TABLE Teaches( fid INTEGER, serno
CHAR(15), semester CHAR(4), PRIMARY KEY
(serno), FOREIGN KEY (fid) REFERENCES
PROFESSORS, FOREIGN KEY (serno) REFERENCES
COURSES)

• Key of relationship set

CREATE TABLE Teaches_Course( serno INTEGER,
subj VARCHAR(30), cid CHAR(15), fid
CHAR(15), name CHAR(40), PRIMARY KEY
(serno), FOREIGN KEY (fid) REFERENCES
PROFESSORS)
Or embed relationship in many entity set
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11 Relationships

• If you borrow money or have credit, you might
  get
• What are the table options?

Describes
Borrower
CreditReport
ssn
rid
delinquent?
debt
name
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Roles Labeled Edges

• Sometimes a relationship connects the same
  entity, and the entity has more than one role
• This often indicates the need for recursive
  queries

Includes
qty
Assembly
Subpart
id
Parts
name
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DDL for Role Example
CREATE TABLE Parts (Id INTEGER, Name
CHAR(15), PRIMARY KEY (ID) ) CREATE
TABLE Includes (Assembly INTEGER,
Subpart INTEGER, Qty INTEGER,
PRIMARY KEY (Assemb, Sub), FOREIGN KEY
(Assemb) REFERENCES Parts, FOREIGN KEY (Sub)
REFERENCES Parts)
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Roles vs. Separate Entities
Married
Husband
Wife
id
id
name
name
Married
What is the difference between these two
representations?
Husband
Wife
id
Person
name
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ISA Relationships Subclassing(Structurally)

• Inheritance states that one entity is a special
  kind of another entity subclass should be
  member of base class

id
People
name
ISA
Employees
salary
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But How Does this Translateinto the Relational
Model?

• Compare these options
• Two tables, disjoint tuples
• Two tables, disjoint attributes
• One table with NULLs
• Object-relational databases

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Weak Entities

• A weak entity can only be identified uniquely
  using the primary key of another (owner) entity.
• Owner and weak entity sets in a one-to-many
  relationship set, 1 owner many weak entities
• Weak entity set must have total participation

Feeds
People
Pets
name
weeklyCost
name
species
ssn
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Translating Weak Entity Sets

• Weak entity set and identifying relationship set
  are translated into a single table when the
  owner entity is deleted, all owned weak entities
  must also be deleted

CREATE TABLE Feed_Pets ( name VARCHAR(20),
species INTEGER, weeklyCost REAL, ssn
CHAR(11) NOT NULL, PRIMARY KEY (pname, ssn),
FOREIGN KEY (ssn) REFERENCES People, ON
DELETE CASCADE)
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N-ary Relationships

• Relationship sets can relate an arbitrary number
  of entity sets

Student
Project
IndepStudy
Advisor
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Summary of ER Diagrams

• One of the primary ways of designing logical
  schemas
• CASE tools exist built around ER (e.g. ERWin,
  PowerBuilder, etc.)
• Translate the design automatically into DDL, XML,
  UML, etc.
• Use a slightly different notation that is better
  suited to graphical displays
• Some tools support constraints beyond what ER
  diagrams can capture
• Can you get different ER diagrams from the same
  data?

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Schema Refinement Design Theory

• ER Diagrams give us a start in logical schema
  design
• Sometimes need to refine our designs further
• Theres a system and theory for this
• Focus is on redundancy of data
• Lets briefly touch on one key concept in
  preparation for Mondays lecture on normalization

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Not All Designs are Equally Good

• Why is this a poor schema design?
• And why is this one better?

Stuff(sid, name, cid, subj, grade)
Student(sid, name) Course(cid, subj) Takes(sid,
cid, exp-grade)
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Focus on the Bad Design

• Certain items (e.g., name) get repeated
• Some information requires that a student be
  enrolled (e.g., courses) due to the key

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Functional DependenciesDescribe Key-Like
Relationships

• A key is a set of attributes where
• If keys match, then the tuples match
• A functional dependency (FD) is a generalization
• If an attribute set determines another, written A
  ! Bthen if two tuples agree on A, they must
  agree on Bsid ! Address
• What other FDs are there in this data?
• FDs are independent of our schema design choice

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Formal Definition of FDs

• Def. Given a relation scheme R (a set of
  attributes) and subsets X,Y of R
• An instance r of R satisfies FD X ? Y if, for
  any two tuples t1, t2 2 r, t1X t2X
  implies t1Y t2Y
• For an FD to hold for scheme R, it must hold for
  every possible instance of r
• (Can a DBMS verify this? Can we determine this
  by looking at an instance?)

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General Thoughts on Good Schemas

• We want all attributes in every tuple to be
  determined by the tuples key attributes
• What does this say about redundancy?
• But
• What about tuples that dont have keys (other
  than the entire value)?
• What about the fact that every attribute
  determines itself?
• Stay tuned for Monday!