03. Relational Database (System Analysis and Development)

7/29/2019 03. Relational Database (System Analysis and Development)

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Relational database

Relational database is away tomanage data that always usestable (tables) to store data andeach table can be related to theothers using the same column,control by DBMS software


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Database Management System (DBMS)

DBMS contains information about a particular enterprise

Collection of interrelated data

Set of programs to access the data

An environment that is both convenientand efficientto use

Database Applications:

Banking: all transactions Airlines: reservations, schedules

Universities: registration, grades

Sales: customers, products, purchases

Online retailers: order tracking, customized recommendations

Manufacturing: production, inventory, orders, supply chain

Human resources: employee records, salaries, tax deductions

Databases touch all aspects of our lives


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Purpose of Database Systems

In the early days, database applications were built directly on top of

file systems Drawbacks of using file systems to store data:

Data redundancy and inconsistency

Multiple file formats, duplication of information in different files

Difficulty in accessing data

Need to write a new program to carry out each new task

Data isolation multiple files and formats

Integrity problems

Integrity constraints (e.g. account balance > 0) become

buried in program code rather than being stated explicitly Hard to add new constraints or change existing ones


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Purpose of Database Systems (Cont.)

Drawbacks of using file systems (cont.)

Atomicity of updates Failures may leave database in an inconsistent state with partial

updates carried out

Example: Transfer of funds from one account to another shouldeither complete or not happen at all

Concurrent access by multiple users Concurrent accessed needed for performance

Uncontrolled concurrent accesses can lead to inconsistencies

Example: Two people reading a balance and updating it at thesame time

Security problems Hard to provide user access to some, but not all, data

Database systems offer solutions to all the above problems


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Example of a Table


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Attribute Types

Each attribute of a Table has a name

The set of allowed values for each attribute is called the domain of theattribute

Attribute values are (normally) required to be atomic; that is, indivisible

E.g. the value of an attribute can be an account number,but cannot be a set of account numbers

Domain is said to be atomic if all its members are atomic

The special value null is a member of every domain

The null value causes complications in the definition of many operations

We shall ignore the effect of null values in our main presentationand consider their effect later


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Table Instance

Jones

Smith

CurryLindsay

customer_name

MainNorthNorthPark

customer_street

HarrisonRyeRyePittsfield

customer_city

customer

attributes

(or columns)

Tuples/records(or rows)


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Tables are Unordered

Order of tuples (records/rows) is irrelevant (tuples (records/rows) may

be stored in an arbitrary order) Example: accountTable with unordered tuples (records/rows)


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Database

A database consists of multiple Tables

Information about an enterprise is broken up into parts, with each Table storingone part of the information

account: stores information about accounts

depositor: stores information about which customer

owns which account

customer: stores information about customers

Storing all information as a single Table such as

bank(account_number, balance, customer_name, ..)

results in

repetition of information

e.g.,if two customers own an account (What gets repeated?)

the need for null values

e.g., to represent a customer without an account

Normalization theory deals with how to design Table normal schemas


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The customerTable


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The deposi torTable


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Keys

Let K R

Kis a superkey of R if values for Kare sufficient to identify a unique tuple(record) of each possible Table r(R)

by possible r we mean a Table rthat could exist in the enterprise weare modeling.

Example: {customer_name, customer_street}

and

{customer_name}

are both superkeys of Customer, if no two customers can possibly have

the same name

In real life, an attribute such as customer_idwould be used instead of

customer_name to uniquely identify customers, but we omit it to keep

our examples small, and instead assume customer names are unique.


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Keys (Cont.)

Kis a candidate key if Kis minimal

Example: {customer_name} is a candidate key for Customer, since itis a superkey and no subset of it is a superkey.

Primary key: a candidate key chosen as the principal means of

identifying tuples (records/rows) within a Table

Should choose an attribute whose value never, or very rarely,changes.

E.g. email address is unique, but may change


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Foreign Keys

A Table schema may have an attribute that corresponds to the primary

key of another Table. The attribute is called a foreign key. E.g. customer_name and account_numberattributes of depositorare

foreign keys to customerand accountrespectively.

Only values occurring in the primary key attribute of the referencedTable may occur in the foreign key attribute of the referencing Table.

Schema diagram


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Query Languages

Language in which user requests information from the database.

Categories of languages Procedural

Non-procedural, or declarative

Pure languages:

Relatioal algebra tuple (record) Relational calculus

Domain Relational calculus

Pure languages form underlying basis of query languages that peopleuse.


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Banking Example

branch (branch_name, branch_city, assets)

customer (customer_name, customer_street, customer_city)

account (account_number, branch_name, balance)

loan (loan_number, branch_name, amount)

depositor (customer_name, account_number)

borrower(customer_name, loan_number)


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Example Queries

Find all loans of over $1200

Find the loan number for each loan of an amount greater than

$1200

amount> 1200 (loan)

loan_number(amount> 1200 (loan))

Find the names of all customers who have a loan, an account, or both,

from the bank

customer_name (borrower) customer_name (depositor)


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Example Queries

Find the names of all customers who have a loan at the Perryridge

branch.

Find the names of all customers who have a loan at thePerryridge branch but do not have an account at any branch ofthe bank.

customer_name (branch_name = Perryridge

(borrower.loan_number = loan.loan_number(borrower x loan)))

customer_name(depositor)

customer_name (branch_name=Perryridge

(borrower.loan_number = loan.loan_number(borrower x loan)))


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Example Queries

Find the names of all customers who have a loan at the Perryridge branch.

Query 2

customer_name(loan.loan_number = borrower.loan_number (

(branch_name = Perryridge (loan)) x borrower))

Query 1

customer_name (branch_name = Perryridge(

borrower.loan_number = loan.loan_number (borrower x loan)))


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Example Queries

Find the largest account balance

Strategy:

Find those balances that are notthe largest

Rename accountrelation as dso that we can compare eachaccount balance with all others

Use set difference to find those account balances that were notfoundin the earlier step.

The query is:

balance(account) - account.balance

(account.balance < d.balance(accountxrd(account)))


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Bank Example Queries

Find the names of all customers who have a loan and an account at

bank.

customer_name (borrower) customer_name (depositor)

Find the name of all customers who have a loan at the bank and the

loan amount

customer_name, loan_number, amount(borrower loan)


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Query 1

customer_name (branch_name = Downtown (depositor account))

customer_name (branch_name = Uptown (depositor account))

Query 2

customer_name, branch_name(depositor account)

rtemp(branch_name)({(Downtown), (Uptown)})

Note that Query 2 uses a constant relation.


Find all customers who have an account from at least the Downtown

and the Uptown branches.


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Find all customers who have an account at all branches located in

Brooklyn city.


customer_name, branch_name(depositor account)

branch_name (branch_city= Brooklyn (branch))


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Extended Relational-Algebra-Operations

Generalized Projection

Aggregate Functions Outer Join


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Generalized Projection

Extends the projection operation by allowing arithmetic functions to be

used in the projection list.

Eis any relational-algebra expression

Each of F1, F2, ,

Fn are are arithmetic expressions involving constantsand attributes in the schema of E.

Given relation credit_info(customer_name, limit, credit_balance), findhow much more each person can spend:

customer_name, limit credit_balance (credit_info)

)(,...,,21

EnFFF


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Aggregate Functions and Operations

Aggregation function takes a collection of values and returns a single

value as a result.avg: average valuemin: minimum valuemax: maximum valuesum: sum of valuescount: number of values

Aggregate operation in relational algebra

Eis any relational-algebra expression

G1, G2, Gn is a list of attributes on which to group (can be empty) Each Fiis an aggregate function

EachAiis an attribute name

)()(,,(),(,,, 221121E

nnn AFAFAFGGG


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Null Values

It is possible for tuples (records/rows) to have a null value, denoted by

null, for some of their attributes nullsignifies an unknown value or that a value does not exist.

The result of any arithmetic expression involving nullis null.

Aggregate functions simply ignore null values (as in SQL)

For duplicate elimination and grouping, null is treated like any othervalue, and two nulls are assumed to be the same (as in SQL)


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Null Values

Comparisons with null values return the special truth value: unknown

If false was used instead of unknown, then not (A < 5)would not be equivalent to A >= 5

Three-valued logic using the truth value unknown:

OR: (unknownortrue) = true,(unknownorfalse) = unknown

(unknown or unknown) = unknown AND: (true and unknown) = unknown,

(false and unknown) = false,(unknown and unknown) = unknown

NOT: (not unknown) = unknown

In SQL P is unknownevaluates to true if predicate P evaluates tounknown

Result of select predicate is treated as false if it evaluates to unknown


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Modification of the Database

The content of the database may be modified using the following

operations: Deletion

Insertion

Updating

All these operations are expressed using the assignment

operator.


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Deletion

A delete request is expressed similarly to a query, except

instead of displaying tuples (records/rows) to the user, theselected tuples (records/rows) are removed from the database.

Can delete only whole tuples (records/rows); cannot deletevalues on only particular attributes

A deletion is expressed in relational algebra by:

rrEwhere ris a relation and Eis a relational algebra query.


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Deletion Examples

Delete all account records in the Perryridge branch.

Delete all accounts at branches located in Needham.

r1branch_city = Needham(account branch )

r2account_number,branch_name, balance (r1)

r3customer_name, account_number(r2 depositor)

account accountr2

depositor depositorr3

Deleteall loan records with amount in the range of 0 to 50

loan loanamount0and amount50 (loan)

accountaccountbranch_name = Perryridge(account)


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Updating

A mechanism to change a value in a tuple (record) without charging all

values in the tuple (record) Use the generalized projection operator to do this task

Each Fiis either

the I th attribute of r, if the I th attribute is not updated, or,

if the attribute is to be updated Fi is an expression, involving onlyconstants and the attributes of r, which gives the new value for theattribute

)(,,,, 21rr

lFFF


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Update Examples

Make interest payments by increasing all balances by 5 percent.

Pay all accounts with balances over $10,000 6 percent interestand pay all others 5 percent

account account_number, branch_name, balance * 1.06(BAL 10000 (account)) account_number, branch_name, balance *1.05 (BAL 10000

(account))

accountaccount_number, branch_name, balance * 1.05(account)


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Home Work

Develop the following tabeles

The branch


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The branch


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The customers

A


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Accounts

Th


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The loan

Th


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The borrower


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Develop following Queries

What is the result of the follwing queries Loan number and the amount of the loan

Names of all customers who have either an account or an loan

Customers with an account but no loan

Largest account balance in the bank

Customers with both an account and a loan at the bank

Documents

03. Relational Database (System Analysis and Development)