CPSC 310 Database Systems

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CPSC 310 Database Systems

• Lecturer Anxiao (Andrew) Jiang
• Lecture One Introduction, E/R Model

http//faculty.cs.tamu.edu/ajiang/310.html
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Textbook and References

• Database Systems, The Complete Book, by Hector
  Garcia-Molina, Jeffrey D. Ullman, and Jennifer
  Widom.
• Slides by Jeffrey Ullman
• http//www-db.stanford.edu/ullman/dscb/pslides/p
  slides.html
• Slides by Hector Garcia-Molina
• http//www-db.stanford.edu/hector/cs245/notes.ht
  m
• Slides by Arthur Keller http//www.soe.ucsc.edu/c
  lasses/cmps180/Winter02
• Slides by Rada Chirkova http//courses.ncsu.edu/c
  sc540/lec/001/
• Slides by Jennifer Welch http//faculty.cs.tamu.e
  du/welch/teaching/310.f05/

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Database Management System (DBMS)

• Specialized software that manages access to very
  large amounts of data
• data persists over a long time
• efficient access
• concurrent access
• reliable and predictable access
• convenient interface language
• Examples Airline reservation systems, banking,
  corporate records, etc.

Slide by Prof. Jennifer Welch, with modification.
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Relational Model

• Store information in tables
• Each table is a relation
• Each column is named with an attribute
• Each row is a tuple
• Example relation named Accounts

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Structured Query Language (SQL) Preview
Accounts
SELECT accountNo FROM Accounts WHERE type
'savings' AND balance lt 0
SELECT balance FROM Accounts WHERE accountNo
67890
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Setting up a Database

• Database schema specifies what relations are in
  the database
• Schema is altered using DDL (data definition
  language) commands
• Only database administrator should be able to do
  this
• Schema affects how the data is stored

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Querying the Database

• A query is expressed in a DML (data manipulation
  language), e.g. SQL
• The query compiler translates query into a query
  plan (sequence of operations to be performed).
• Query compilation includes query optimization, to
  choose best sequence of operations
• Execution engine performs the operations,
  interacting with

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Storage and Buffer Management

• Data usually is stored on disk, in units called
  blocks
• Storage manager controls movement of data between
  disk and main memory
• Buffer manager controls partitioning of main
  memory into block-sized regions
• Relies on info about data, schema, statistics,
  and indexes (special data structures for
  efficient access)

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Transactions

• Usually database queries are grouped into
  transactions
• A transaction must satisfy ACID properties
• Atomicity either all the changes or none of them
  are made to the database
• Consistency transaction must preserve
  consistency constraints of the database
• Isolation the result must be "as if" the
  transaction ran alone
• Durability once transaction has completed,
  changes must not be lost

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Transaction Processing

• Every change is logged separately on disk by log
  manager
• After a system failure, recovery manager uses log
  on disk to reconstruct a consistent state
• Scheduler ensures that concurrently executing
  transactions do not interfere with each other

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Outline of Course

• Database Design
• Entity-Relationship model notation to express
  relationships among the data (Ch 2)
• Relational model easier to implement than E-R
  (Ch 3)
• Database Programming
• Relational algebra how to operate on relations
  (Ch 5)
• Programming with SQL (Chs 6-8)
• Database Implementation
• storage management (Chs 11-13)
• query processing (Chs 15-16)
• transaction processing (Chs 17-19)

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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 Candies has two attributes, name and
  manf (manufacturer).
• Each Candies entity has values for these two
  attributes, e.g. (Twizzlers, Hershey)

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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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Relationship Set

• The current value of an entity set is the set
  of entities that belong to it.
• Example the set of all stores 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

Store Candy 7-11 Twizzlers 7-11 Kitkat Kroger
Twizzlers Kroger Snickers Kroger Reeses
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Multiway Relationships

• Sometimes, we need a relationship that connects
  more than two entity sets.
• Suppose that consumers will only buy certain
  candies at certain stores.
• Our three binary relationships Likes, Sells, and
  Frequents do not allow us to make this
  distinction.
• But a 3-way relationship would.

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Example
name
addr
name
manf
Stores
Candies
license
Preferences
Con- sumers
name
addr
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A Typical Relationship Set
Store Consumer Candy 7-11 Ann Kitkat Kroger An
n Twizzler Kroger Ann Snickers 7-11 Bob Twizz
ler 7-11 Bob Kitkat 7-11 Cal Kitkat Kroger Ca
l Reeses
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Many-Many Relationships

• Focus binary relationships, such as Sells
  between Stores and Candies.
• In a many-many relationship, an entity of either
  set can be connected to many entities of the
  other set.
• E.g., a store sells many candies a candy is sold
  by many stores.

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In Pictures
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
many-one
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Example

• Favorite, from Consumers to Candies is many-one.
• A consumer has at most one favorite candy.
• But a candy can be the favorite of any number of
  consumers, 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 Candies.
• A candy 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
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
Likes
Con- sumers
Candies
Favorite
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Example

• Consider Best-seller between Manfs and Candies.
• Some candies are not the best-seller of any
  manufacturer, so a rounded arrow to Manfs would
  be inappropriate.
• But a candy manufacturer has to have a
  best-seller.

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In the E/R Diagram
Best- seller
Manfs
Candies
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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
Sells
Stores
Candies
price
Price is a function of both the candy and the
store, 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
Sells
Stores
Candies
Note convention arrow from multiway
relationship all other entity sets together
determine a unique one of these.
Prices
price
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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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Example
Relationship Set Buddy1 Buddy2 Bob
Ann Joe Sue Ann Bob Joe
Moe
Buddies
1
2
Con- sumers
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Subclasses

• Subclass special case fewer entities more
  properties.
• Example Chocolates are a kind of candy.
• Not every candy is a chocolate, but some are.
• Let us suppose that in addition to all the
  properties (attributes and relationships) of
  candies, chocolates also have the attribute color
  (dark, white or light).

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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
Candies
name
manf
isa
Choc- olates
color
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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
Candies
name
manf
isa
Choc- olates
color
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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 Candies
Candies
name
manf
isa
Choc- olates
color
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Example a Multi-attribute Key
dept
number
hours
room
Courses

• Note that hours and room could also serve as a
• key, but we must select only one key.

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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
name
name
number
Plays- on
Players
Teams

• 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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Design Techniques

• Avoid redundancy.
• Limit the use of weak entity sets.
• Dont 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
name
name
addr
ManfBy
Candies
Manfs
This design gives the address of each
manufacturer exactly once.
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Example Bad
name
name
addr
ManfBy
Candies
Manfs
manf
This design states the manufacturer of a candy
twice as an attribute and as a related entity.
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Example Bad
name
manf
manfAddr
Candies

• This design
• repeats the manufacturers address once for each
  candy and
• loses the address if there are temporarily no
  candies for a manufacturer.

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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
name
name
addr
ManfBy
Candies
Manfs

• Manfs deserves to be an entity set because of
  the nonkey attribute addr.
• Candies deserves to be an entity set because it
  is the many of the many-one relationship
  ManfBy.

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Example Good
name
manf
Candies
There is no need to make the manufacturer an
entity set, because we record nothing about
manufacturers besides their name.
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Example Bad
name
name
ManfBy
Candies
Manfs
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.
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Dont 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 IDs for
  entity sets.
• Examples include social-security numbers,
  automobile VINs 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 IDs.
• Example it is unlikely that there could be an
  agreement to assign unique player numbers across
  all football teams in the world.