Managing external data Part 1 Design of Databases
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Gitte ChristensenDyalog Ltd
Managing external data Part 1
Design of Databases
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Purpose• To give you a crash course in data analysis
and databases• After part 1 Design of Databases you will
be able to analyse and organise data based on a requirement spec or use case.
• After part 2 Database programming you will be able to use relational data in your APL applications
• After part 3 Database Implementation you will be able to choose between different storage methods based on structure and use of data and performance considerations
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Agenda
• The Relational Model– Entity/Relation model– Convert E/R to table structure– Relational Algebra
• Semistructured data• Multidimensional data
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Data Models
• A Database models some portion of the real world.
• Data Model is link between user’s view of the world and bits stored in computer.
• We will concentrate on the Relational Model
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Data Models• A data model is a collection of concepts
for describing data.• A database schema is a description of a
particular collection of data, using a given data model.
• The relational model of data is the most widely used model today.– Main concept: relation, basically a
table with rows and columns.– Every relation has a schema, which
describes the columns, or fields.
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Levels of Abstraction• Views describe how
users see the data.
• Conceptual schema defines logical structure
• Physical schema describes the files and indexes used.
• (sometimes called the ANSI/SPARC model)
Physical Schema
Conceptual Schema
View 1 View 2 View 3
DB
Users
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Data Independence• A Simple Idea:
Applications should be insulated from how data is structured and stored.
• Logical data independence: Protection from changes in logical structure of data.
• Physical data independence: Protection from changes in physical structure of data.
Physical Schema
Conceptual Schema
View 1 View 2 View 3
DB
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Entity-Relationship Model
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Purpose of E/R Model
• The E/R model allows us to sketch database designs.– Kinds of data and how they connect.– Not how data changes.
• Designs are pictures called entity-relationship diagrams.
• Later: convert E/R designs to relational DB designs.
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Entity Sets
• Entity = “thing” or object.• Entity set = collection of similar entities.
– Similar to a class in object-oriented languages.
• Attribute = property of (the entities of) an entity set.– Attributes are simple values, e.g. integers or
character strings.
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E/R Diagrams
• In an entity-relationship diagram:
– Entity set = rectangle.
– Attribute = oval, with a line to the rectangle representing its entity set.
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Example
• Entity set Beers has two attributes, name and manf (manufacturer).
• Each Beers entity has values for these two attributes, e.g. (Bud, Anheuser-Busch)
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Beers
name manf
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Relationships• A relationship connects two or more
entity sets.• It is represented by a diamond, with
lines to each of the entity sets involved.
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Example
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Drinkers addrname
Beers
manfname
Bars
name
license
addr
Note:license =beer, full,none
Sells Bars sell somebeers.
Likes
Drinkers likesome beers.Frequents
Drinkers frequentsome bars.
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Relationship Set• The current “value” of an entity set is
the set of entities that belong to it.– Example: the set of all bars in our
database.
• The “value” of a relationship is a set of lists of currently related entities, one from each of the related entity sets.
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Example• For the relationship Sells, we might
have a relationship set like:
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Bar BeerJoe’s Bar BudJoe’s Bar MillerSue’s Bar BudSue’s Bar Pete’s AleSue’s Bar Bud Lite
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Case Movie Database• We want to create a movie database
which will allow our users to find information about movies
• Each movie has a title, a production year, lenght in minutes, whether it is color or b/w and an owner, a studio
• We have adresses for the studios and the actors
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EntityName
AttriAbuteAttributeName
Relationship Draw a model of the Movies database using these symbols
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Multiway Relationships• Sometimes, we need a relationship
that connects more than two entity sets.
• 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.19
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Example
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Bars Beers
Drinkers
name nameaddr manf
name addr
license
Preferences
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A Typical Relationship Set
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Bar Drinker BeerJoe’s Bar Ann MillerSue’s Bar Ann BudSue’s Bar Ann Pete’s AleJoe’s Bar Bob BudJoe’s Bar Bob MillerJoe’s Bar Cal MillerSue’s Bar Cal Bud Lite
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Case Movie Database• In each movie there are actors who
are contracted by the studios• Add this relationship to your model
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Many-Many Relationships• Focus: binary relationships, such as
Sells between Bars and Beers.• In a many-many relationship, an
entity of either set can be connected to many entities of the other set.– E.g., a bar sells many beers; a beer is
sold by many bars.
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In Pictures:
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many-many
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Many-One Relationships• Some binary relationships are many -
one from one entity set to another.• Each entity of the first set is
connected to at most one entity of the second set.
• But an entity of the second set can be connected to zero, one, or many entities of the first set.
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In Pictures:
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many-one
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Example
• Favorite, from Drinkers to Beers is many-one.
• A drinker has at most one favorite beer.
• But a beer can be the favorite of any number of drinkers, including zero.
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One-One Relationships
• In a one-one relationship, each entity of either entity set is related to at most one entity of the other set.
• Example: Relationship Best-seller between entity sets Manfs (manufacturer) and Beers.– A beer cannot be made by more than one
manufacturer, and no manufacturer can have more than one best-seller (assume no ties).
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In Pictures:
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one-one
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Representing “Multiplicity”• Show a many-one relationship by an
arrow entering the “one” side.• Show a one-one relationship by
arrows entering both entity sets.• Rounded arrow = “exactly one,” i.e.,
each entity of the first set is related to exactly one entity of the target set.
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Example
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Drinkers BeersLikes
Favorite
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Example• Consider Best-seller between Manfs
and Beers.• Some beers are not the best-seller of
any manufacturer, so a rounded arrow to Manfs would be inappropriate.
• But a beer manufacturer has to have a best-seller.
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In the E/R Diagram
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Manfs BeersBest-seller
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Case Movie Database• Add arrows to your diagram so it
reflects the kind of relations between the entities
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Attributes on Relationships• Sometimes it is useful to attach an
attribute to a relationship.• Think of this attribute as a property
of tuples in the relationship set.
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Example
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Bars BeersSells
price
Price is a function of both the bar and the beer,not of one alone.
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Equivalent Diagrams Without Attributes on Relationships• Create an entity set representing
values of the attribute.• Make that entity set participate in
the relationship.
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Example
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Bars BeersSells
price
PricesNote convention: arrowfrom multiway relationship= “all other entity setstogether determine aunique one of these.”
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Roles• Sometimes an entity set appears
more than once in a relationship.• Label the edges between the
relationship and the entity set with names called roles.
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Example
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Drinkers
Married
husband wife
Relationship Set
Husband WifeBob AnnJoe Sue… …
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Example
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Drinkers
Buddies
1 2
Relationship Set
Buddy1 Buddy2Bob AnnJoe SueAnn BobJoe Moe… …
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Case Movie Database• The actors can be contracted either
by the studio producing the movie or by another studio who rents the actor to the producing studio
• We would like to record what the actor is paid for appearing in a movie
• Update your model to reflect the new facts
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Subclasses• Subclass = special case = fewer
entities = more properties.• Example: Ales are a kind of beer.
– Not every beer is an ale, but some are.– Let us suppose that in addition to all the
properties (attributes and relationships) of beers, ales also have the attribute color.
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Subclasses in E/R Diagrams• Assume subclasses form a tree.
– I.e., no multiple inheritance.
• Isa triangles indicate the subclass relationship.– Point to the superclass.
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Example
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Beers
Ales
isa
name manf
color
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Case Movie Database• For some movies like cartoons we
have a different kind of actor, voices.• Design a subclass to reflect this fact
ISA
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E/R Vs. Object-Oriented Subclasses• In OO, objects are in one class only.
– Subclasses inherit from superclasses.
• In contrast, E/R entities have representatives in all subclasses to which they belong.– Rule: if entity e is represented in a
subclass, then e is represented in the superclass.
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Example
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Beers
Ales
isa
name manf
color
Pete’s Ale
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Keys• A key is a set of attributes for one
entity set such that no two entities in this set agree on all the attributes of the key.– It is allowed for two entities to agree on
some, but not all, of the key attributes.
• We must designate a key for every entity set.
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Keys in E/R Diagrams
• Underline the key attribute(s).• In an Isa hierarchy, only the root
entity set has a key, and it must serve as the key for all entities in the hierarchy.
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Example: name is Key for Beers
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Beers
Ales
isa
name manf
color
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Example: a Multi-attribute Key
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Courses
dept number hours room
• Note that hours and room could also serve as a key, but we must select only one key.
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Case Movie Database• Add keys to your diagram
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Weak Entity Sets• Occasionally, entities of an entity set
need “help” to identify them uniquely.• Entity set E is said to be weak if in
order to identify entities of E uniquely, we need to follow one or more many-one relationships from E and include the key of the related entities from the connected entity sets.
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Example
• name is almost a key for football players, but there might be two with the same name.
• number is certainly not a key, since players on two teams could have the same number.
• But number, together with the team name related to the player by Plays-on should be unique.
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In E/R Diagrams
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Players TeamsPlays-on
name namenumber
• Double diamond for supporting many-one relationship.• Double rectangle for the weak entity set.
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Weak Entity-Set Rules• A weak entity set has one or more many-
one relationships to other (supporting) entity sets.– Not every many-one relationship from a weak
entity set need be supporting.
• The key for a weak entity set is its own underlined attributes and the keys for the supporting entity sets.– E.g., (player) number and (team) name is a key
for Players in the previous example.
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Case Movie Database• We would like to record which
camera crews shot a particular movie• Camera crews are numbered within
each studio• Add these facts to your diagram
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Design Techniques1. Avoid redundancy.2. Limit the use of weak entity sets.3. Don’t use an entity set when an
attribute will do.
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Avoiding Redundancy
• Redundancy occurs when we say the same thing in two or more different ways.
• Redundancy wastes space and (more importantly) encourages inconsistency.– The two instances of the same fact may
become inconsistent if we change one and forget to change the other.
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Example: Good
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Beers ManfsManfBy
name
This design gives the address of each manufacturer exactly once.
name addr
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Example: Bad
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Beers ManfsManfBy
name
This design states the manufacturer of a beer twice: as an attribute and as a related entity.
name
manf
addr
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Example: Bad
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Beers
name
This design repeats the manufacturer’s address once for each beer and loses the address if there are temporarily no beers for a manufacturer.
manf manfAddr
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Entity Sets Versus Attributes• An entity set should satisfy at least
one of the following conditions:– It is more than the name of something;
it has at least one nonkey attribute.or
– It is the “many” in a many-one or many-many relationship.
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Example: Good
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Beers ManfsManfBy
name
•Manfs deserves to be an entity set because of the nonkey attribute addr.•Beers deserves to be an entity set because it is the “many” of the many-one relationship ManfBy.
name addr
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Example: Good
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Beers
name
There is no need to make the manufacturer an entity set, because we record nothing about manufacturers besides their name.
manf
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Example: Bad
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Beers ManfsManfBy
name
Since the manufacturer is nothing but a name, and is not at the “many” end of any relationship, it should not be an entity set.
name
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Don’t Overuse Weak Entity Sets• Beginning database designers often doubt
that anything could be a key by itself.– They make all entity sets weak, supported by
all other entity sets to which they are linked.
• In reality, we usually create unique ID’s for entity sets.– Examples include social-security numbers,
automobile VIN’s etc.
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When Do We Need Weak Entity Sets?
• The usual reason is that there is no global authority capable of creating unique ID’s.
• Example: it is unlikely that there could be an agreement to assign unique player numbers across all football teams in the world.
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Break
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How to translate ER Model to Relational Model
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Concepts
Relational Model is made up of tables
• A row of table = a relational instance/tuple
• A column of table = an attribute• A table = a schema/relation• Cardinality = number of rows• Degree = number of columns
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Example
SID Name Major GPA
1234 John CS 2.8
5678 Mary EE 3.6
tuple/relational instance
Attribute
4 Degree
Cardinality =
2
A Schema / Relation
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From ER Model to Relational ModelSo… how do we convert an ER diagram into
a table?? Simple!!Basic Ideas:• Build a table for each entity set• Build a table for each relationship set if necessary
(more on this later)• Make a column in the table for each attribute in
the entity set• Indivisibility Rule and Ordering Rule• Primary Key
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Example – Strong Entity Set
SID Name Major GPA
1234 John CS 2.8
5678 Mary EE 3.6
Student
SID Name
Major GPA
Advisor Professor
SSN Name
Dept
SSN Name Dept
9999 Smith Math
8888 Lee CS
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Representation of Weak Entity Set• Weak Entity Set Cannot exists alone• To build a table/schema for weak entity
set – Construct a table with one column for each
attribute in the weak entity set– Remember to include discriminator– Augment one extra column on the right side of
the table, put in there the primary key of the Strong Entity Set (the entity set that the weak entity set is depending on)
– Primary Key of the weak entity set = Discriminator + foreign key
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Example – Weak Entity Set
Age Name Parent_SID
10 Bart 1234
8 Lisa 5678
Student
SID Name
Major GPA
NameAge
Childrenowns
Primary key of Children is Parent_SID + Name
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Representation of Relationship Set
--This is a little more complicated--• Unary/Binary Relationship set
– Depends on the cardinality and participation of the relationship
– Two possible approaches
• N-ary (multiple) Relationship set– Primary Key Issue
• Identifying Relationship– No relational model representation necessary
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Representing Relationship SetUnary/Binary Relationship• For one-to-one relationship w/out total participation
– Build a table with two columns, one column for each participating entity set’s primary key. Add successive columns, one for each descriptive attributes of the relationship set (if any).
• For one-to-one relationship with one entity set having total participation– Augment one extra column on the right side of the
table of the entity set with total participation, put in there the primary key of the entity set without complete participation as per to the relationship.
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Example – One-to-One Relationship Set
SID Maj_ID Co S_Degree
9999 07 1234
8888 05 5678
Student
SID Name
Major GPA
ID Code
Majorstudy
Primary key can be either SID or Maj_ID_Co
Degree
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Example – One-to-One Relationship Set
SID Name Major GPA LP_S/N Hav_Cond
9999 Bart Economy -4.0 123-456 Own
8888 Lisa Physics 4.0 567-890 Loan
Student
SID Name
Major GPA
S/N #
LaptopHave
* Primary key can be either SID or LP_S/N
Condition
Brand
1:1 Relationship
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Representing Relationship SetUnary/Binary Relationship• For one-to-many relationship w/out total
participation – Same thing as one-to-one
• For one-to-many/many-to-one relationship with one entity set having total participation on “many” side– Augment one extra column on the right
side of the table of the entity set on the “many” side, put in there the primary key of the entity set on the “one” side as per to the relationship.
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Example – Many-to-One Relationship Set
SID Name Major GPA Pro_SSN Ad_Sem
9999 Bart Economy -4.0 123-456 Fall 2006
8888 Lisa Physics 4.0 567-890 Fall 2005
Student
SID Name
Major GPA
SSN
Professor
* Primary key of this table is SID
Semester
Name
N:1 Relationship
Dept
Advisor
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Representing Relationship SetUnary/Binary Relationship• For many-to-many relationship
– Same thing as one-to-one relationship without total participation.
– Primary key of this new schema is the union of the foreign keys of both entity sets.
– No augmentation approach possible…
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Representing Relationship SetN-ary Relationship• Intuitively Simple
– Build a new table with as many columns as there are attributes for the union of the primary keys of all participating entity sets.
– Augment additional columns for descriptive attributes of the relationship set (if necessary)
– The primary key of this table is the union of all primary keys of entity sets that are on “many” side
– That is it, we are done.
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Example – N-ary Relationship Set
P-Key1 P-Key2 P-Key3 A-Key D-Attribute
9999 8888 7777 6666 Yes
1234 5678 9012 3456 No
E-Set 1
P-Key1
Another Set
* Primary key of this table is P-Key1 + P-Key2 + P-Key3
D-Attribute
A relationship
A-Key
E-Set 2
P-Key2
E-Set 3
P-Key3
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Representing Relationship SetIdentifying Relationship• This is what you have to know
– You DON’T have to build a table/schema for the identifying relationship set once you have built a table/schema for the corresponding weak entity set
– Reason:• A special case of one-to-many with total participation• Reduce Redundancy
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Representing Composite Attribute• Relational Model Indivisibility Rule Applies• One column for each component attribute• NO column for the composite attribute itself
Professor
SSN Name
Address
SSN Name Street City
9999 Dr. Smith 50 1st St. Fake City
8888 Dr. Lee 1 B St. San Jose
Street City
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Representing Multivalue Attribute• For each multivalue attribute in an entity
set/relationship set– Build a new relation schema with two columns– One column for the primary keys of the entity
set/relationship set that has the multivalue attribute
– Another column for the multivalue attributes. Each cell of this column holds only one value. So each value is represented as an unique tuple
– Primary key for this schema is the union of all attributes
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Example – Multivalue attribute
SID Name Major GPA
1234 John CS 2.8
5678 Homer EE 3.6
Student
SID Name
Major GPA
Stud_SID Children
1234 Johnson
1234 Mary
5678 Bart
5678 Lisa
5678 Maggie
Children
The primary key for this table is Student_SID + Children, the union of all attributes
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Representing Class Hierarchy• Two general approaches depending on
disjointness and completeness– For non-disjoint and/or non-complete class hierarchy:
• create a table for each super class entity set according to normal entity set translation method.
• Create a table for each subclass entity set with a column for each of the attributes of that entity set plus one for each attributes of the primary key of the super class entity set
• This primary key from super class entity set is also used as the primary key for this new table
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Example
SSN SID Status Major GPA
1234 9999 Full CS 2.8
5678 8888 Part EE 3.6
Student
SID Status
Major GPA
SSN Name Gender
1234 Homer Male
5678 Marge Female
Person
Gender
SSN Name
ISA
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Case Movie Database• Convert your E/R diagram to
relational tables
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Relational Algebra
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Relational Algebra
Relational Algebra is :•the formal description of how a relational database operates•the mathematics which underpin SQL operations.
Operators in relational algebra are not necessarily the same as SQL operators, even if they have the same name.
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Terminology
• Relation - a set of tuples.• Tuple - a collection of attributes which
describe some real world entity.• Attribute - a real world role played by a
named domain.• Domain - a set of atomic values.• Set - a mathematical definition for a
collection of objects which contains no duplicates.
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Operators - Write
• INSERT - provides a list of attribute values for a new tuple in a relation. This operator is the same as SQL.
• DELETE - provides a condition on the attributes of a relation to determine which tuple(s) to remove from the relation. This operator is the same as SQL.
• MODIFY - changes the values of one or more attributes in one or more tuples of a relation, as identified by a condition operating on the attributes of the relation. This is equivalent to SQL UPDATE.
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Operators - Retrieval
There are two groups of operations:• Mathematical set theory based relations:
UNION, INTERSECTION, DIFFERENCE, and CARTESIAN PRODUCT.
• Special database operations: SELECT (not the same as SQL SELECT), PROJECT, and JOIN.
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Relational SELECT
SELECT is used to obtain a subset of the tuples of a relation that satisfy a select condition.For example, find all employees born after 1st Jan 1950:
SELECT dob > ’01/JAN/1950’ (employee)
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Relational PROJECTThe PROJECT operation is used to select a subset of the attributes of a relation by specifying the names of the required attributes.For example, to get a list of all employees surnames and employee numbers:
PROJECT surname,empno (employee)
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SELECT and PROJECT
FROM employeeWHERE depno = 1;
SELECT empno
PROJECT empno (SELECT depno = 1 (employee))
Mapping this back to SQL gives:
SELECT and PROJECT can be combined together. For example, to get a list of employee numbers for employees in department number 1:
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Set Operations - semanticsConsider two relations R and S.• UNION of R and S
the union of two relations is a relation that includes all the tuples that are either in R or in S or in both R and S. Duplicate tuples are eliminated.
• INTERSECTION of R and Sthe intersection of R and S is a relation that includes all tuples that are both in R and S.
• DIFFERENCE of R and Sthe difference of R and S is the relation that contains all the tuples that are in R but that are not in S.
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SET Operations - requirementsFor set operations to function correctly the relations R and S must be union compatible. Two relations are union compatible if
– they have the same number of attributes– the domain of each attribute in column
order is the same in both R and S.
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UNION Example
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INTERSECTION Example
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DIFFERENCE Example
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CARTESIAN PRODUCT
The Cartesian Product is also an operator which works on two sets. It is sometimes called the CROSS PRODUCT or CROSS JOIN.
It combines the tuples of one relation with all the tuples of the other relation.
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CARTESIAN PRODUCT Example
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JOIN OperatorJOIN is used to combine related tuples from two
relations:• In its simplest form the JOIN operator is just the cross
product of the two relations.• As the join becomes more complex, tuples are
removed within the cross product to make the result of the join more meaningful.
• JOIN allows you to evaluate a join condition between the attributes of the relations on which the join is undertaken.
The notation used is R JOIN join condition S
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JOIN Example
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Natural Join
Invariably the JOIN involves an equality test, and thus is often described as an equi-join. Such joins result in two attributes in the resulting relation having exactly the same value. A ‘natural join’ will remove the duplicate attribute(s).– In most systems a natural join will require that the
attributes have the same name to identify the attribute(s) to be used in the join. This may require a renaming mechanism.
– If you do use natural joins make sure that the relations do not have two attributes with the same name by accident.
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OUTER JOINsNotice that much of the data is lost when applying a join to two relations. In some cases this lost data might hold useful information. An outer join retains the information that would have been lost from the tables, replacing missing data with nulls.
There are three forms of the outer join, depending on which data is to be kept.•LEFT OUTER JOIN - keep data from the left-hand table•RIGHT OUTER JOIN - keep data from the right-hand table•FULL OUTER JOIN - keep data from both tables
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OUTER JOIN Example 1
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OUTER JOIN Example 2
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Semistructured data
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Root
cf mh sw
Carrie Fisher
address address
name
city
street
MapleHollywood
LocustMalibu
city
star
starmovie
title year
Star Wars1977
Mark HamillOak
Brentwood
name
street city
street
starIn
starOf
starIn
starOf
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Multidimensional data
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End of Part 1