Week 2 Lecture 2 Structure of a database. External Schema Conceptual Schema Internal Schema Physical Schema.

Week 2 Lecture 2

Structure of a database

Structure of a database

ExternalSchema

ConceptualSchema

InternalSchema

PhysicalSchema

External level

• Level visible to user

• Multiple views of the system– e.g. View an order - see limited product and

customer information

• Only the database Administrator may access the whole database at this level

EXTERNAL SCHEMA

• Each external view is defined by means of an external schema

• Provides definitions of each external view.• Written in a Data Definition Language• individual to the user • accessed through a 3GL, a query language or a

special purpose forms or menu-based language

Conceptual level

• CONCEPTUAL - represents the entire information content of the database

• Consists of multiple types of conceptual record. This level preserves the data independence of the database.

• CONCEPTUAL SCHEMA - defines each of the various types of conceptual record, in a conceptual Data Definition Language.

Internal level

• INTERNAL - a low-level representation of the entire database; it consists of multiple occurrences of multiple types of internal record. It is the stored record, inasmuch as it contains all but the device-specific information on the storage of the database.

• PHYSICAL - the physical device and block addresses for each of the records.

Mappings

• Each level maps onto adjoining levels• conceptual / internal mapping specifies how

conceptual records and fields are represented at the internal level

• Changes can be made in the internal level without affecting the conceptual level

• external / conceptual mapping defines the correspondence between an external view and the conceptual view

DBMS - Database Management System

• software handling access to the database

• allows both the database administrator and all users the access to the database to which they are entitled

How requests are processed

• User issues request (e.g. through SQL)• DBMS intercepts and analyses request• DBMS inspects user's external schema, external

to conceptual mapping, conceptual schema, conceptual to internal mapping and the storage structure definition.

• DBMS executes operations on stored database.

DATABASE ADMINISTRATOR (DBA)

• Decide on the storage structure and access strategy

• Liaise with the users

• Define security and integrity checks

• Define a backup and recovery strategy

• Monitor and respond to performance

Utilities used by the DBA

• Load routines

• Dump/Restore routines

• Reorganisation routines

• Statistics routines

• Analysis routines

• Data dictionary (containing METADATA, which gives data descriptions and mappings)

Building a conceptual schema

Please note: The diagrams in this presentation are drawn using a tool

that you will not be using. You will be using ERWin.

Building tables

• Use primary keys• Put the tables with ONLY primary keys in first.

– This is the first layer.• Put the tables that reference those tables in

next. – This is the second layer.– This layer uses the keys of the first layer as

FOREIGN keys.– The second layer cannot be placed until the first layer

is complete.• See the BUILDER example following:

Required new data structure

COrder

CorderNoOrder Date

* Customer Id* StaffPaid* StaffIssued

COrderLine

QuantityRequiredCOrderNoStock Code

Customer

Customer IdCustomer NameCustomer Address

SOrder

SupplierOrderNoSupplierOrderDateDeliveredDate

* Supplier Id

SOrderLine

StockRequiredSupplierOrderNoStock Code

Staff

Staff noStaff nameStaff role

Stock

Stock CodeStock DescriptionUnit PriceUnitCostPriceStock levelReorder level

* Supplier Id

Supplier

Supplier IdSupplier NameSupplier AddressAmount Owed

COrder

CorderNoOrder Date

* Customer Id* StaffPaid* StaffIssued

COrderLine

QuantityRequiredCOrderNoStock Code

Customer

Customer IdCustomer NameCustomer Address

SOrder

SupplierOrderNoSupplierOrderDateDeliveredDate

* Supplier Id

SOrderLine

StockRequiredSupplierOrderNoStock Code

Staff

Staff noStaff nameStaff role

Stock

Stock CodeStock DescriptionUnit PriceUnitCostPriceStock levelReorder level

* Supplier Id

Supplier

Supplier IdSupplier NameSupplier AddressAmount Owed

Hierarchy of data structure

Please note

• To show self-joins, the staff table has since been amended:alter table staff add reports_to number(7);

alter table staff add constraint has_as_boss foreign key (reports_to) references staff(staff_no);

• The new structure is the same as the one in the ‘builder schema’ link in your web page and webCT.

Layers of tables

• The tables Customer, Staff and Supplier have only primary keys. They are the foundation layer. Layer 1.

• The tables Corder, Stock and SOrder have foreign keys that only reference the foundation layer. They are Layer 2.

• COrderline and Sorderline depend on the tables in Layer 2. They are layer 3.

Analogous to building bricks

Layer 1

Cus

tom

erC

usto

mer

Sta

ffS

taff

Supp

lier

Supp

lier

The customer table is added, with key CustomerId, the Staff with key StaffNo and the Supplier with key SupplierId.

Layer 2

COrder

Stock

SOrder

CustomerStaffSupplier

The stock and the SOrder depend on the Supplier, both having foreign key SupplierId.

The COrder depends on BOTH Staff and Customer, having foreign keys StaffPaid, StaffIssued and CustomerId.

Layer 3

Supplier Staff Customer

COrderStock

SOrder

SOrderlineSOrderline COrderlineCOrderline

•Both the supplier order line and the order line depend on the stock, having stockcode as a foreign key and part of their key.

•COrderline depends on COrder.

•Sorderline depends on SOrder

The built database

Recap

• Look back at the blocks.– The table creates are the structure or the framework

- i.e. the architect’s drawing– The inserts are like the bricks. You cannot insert

into a table unless there is supporting data in the table on which it depends.

• Do– Creates starting with the one(s) with no dependents– Inserts starting with the one(s) with no dependents– Deletes starting with the one(s) on which no other

depends– Drops starting with the one(s) on which no other

depends

Relational database design

• The conceptual schema is built using CREATE TABLE commands.

• A relation is relational…If and only if every non-key attribute is determined by the KEY… the WHOLE KEY…. and nothing but the KEY so help me CODD!Dr. E.F. Codd, an IBM researcher, first developed the

relational data model in 1970

Bottom-up Approach to Data Modelling

• Objectives– Define the purpose of normalisation– Determinacy / Dependency– Defining the Data Dictionary– First Normal Form– Second Normal Form– Third Normal Form

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Normalisation

• Normalisation provides algorithms for reducing complex data structures into simple data structures

• Concerned with –tidying up the data so there is no data redundancy–ensuring that data is grouped logically

• Bottom up approach - start with data items• Codd’s Law is a set of rules which ensure that

the data is grouped correctly–A normal form is a convenient structure into which

data can be organised

Concept of Determinacy and Dependency

• if A determines B - then B is dependent on A

• B is dependent on A if given a value for A, there is only one possible value for B– e.g. – student name is dependent on student number

and– student number determines student name

Data Dictionary

– central store of data that supports other models– unambiguous and concise way of recording data about

data (metadata)– can be recorded manually or using a software tool– encourages consistency between models by using

same names in different models– prevents duplication of data– resolves problems of aliases which are all recorded

against the appropriate data item– built up as the models are developed– aids communication as everyone on development team

knows the exact meaning of words and terms used

Sequence, selection, iteration

• Sequence : +– CustomerDetails = Name + Address + PhoneNo

• Repetition : { }– Name = Title + {Initial} + Surname

• Optionality : ( )– CustomerDetails = Name + Address +(PhoneNo)

• Selection : […|…]– Name = (Title) + [ {ForeName} | {Initial} ] +

Surname

• Values : “…”– Title = [ “Dr” | “Mr” | “Mrs” | “Ms” ]

• Comments : *….*

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Example

• Full data dictionary entries for CustomerDetailsAddress = {AddressLine}

CustomerDetails = Name + Address + (PhoneNo)

Name = (Title) + [ {ForeName} | {Initial} ] +Surname

Title = [ “Dr” | “Mr” | “Mrs” | “Ms” ]

Two Technique Approach to Data Modelling

• E-R diagramming to find and group all data items

• normalisation to ensure data items are grouped correctly

Before we begin…

• We have already discussed– Entities– Attributes– Values– Data and Metadata

• We now take one of the documents and apply:– Unnormalised form rules (Data Dictionary format)– First Normal Form rules– Second Normal Form rules– Third Normal Form rules