REA analysisand

E-R diagramming

Part IApril 10, 2008

What are we hoping to achieve?

• Tool for designing a database system to meet the needs of the organization – or the system - if you are not considering an

enterprise wide system (ERP).

• REA modeling (ERA modeling, REA analysis, etc.) is a method of analyzing and thinking about the system

• E-R diagramming is a means of diagramming what the database should look like based upon the analysis above.

What are we hoping to achieve?

• What we want to do is follow a structured approach for designing databases.

• The basic element in a database is called an entity -– What do you think an entity might be

relative to an ACCESS database?

Some of the usual suspects…

• Entities• Relations• Events• Resources• Agents• Locations• Concatenated keys• Cardinality

Resource-Event-Agent modeling

• REA modeling is a hot topic in systems circles• It has gone through several name/content

variations– ERA modeling (more of a focus on events - typically

this is the way it is done - but the name is not as easy to remember)

– REAL• Resources• Events• Agents• Locations

• Some books combine REA and E-R diagramming and some make no distinction– IT CAN GET CONFUSING

Resource-Event-Agent(-Location) analysis and modeling

• We focus on events, which are things that get recorded in our system

• For each event we will possibly have– The event itself– Resources consumed or obtained– Internal agents (entities)– External agents (entities)– Perhaps a location

• The reason that the word entities is in parentheses is that with this type of modeling, all five of these things are referred to as “entities”.

• This sounds a lot like narratives, DFDs and flowcharts, huh?

REA analysis

• Think back to the purchase order in the SUA that we looked at a few days ago…

Where

Who

What

Entity-Relationship diagramming

• Sometimes called REA diagramming (a specific form of E-R)

• It uses three symbols– A rectangle

• An entity (but not the same as in DFDs and flowcharts

– A diamond• A relationship

– An oval• An attribute

Entity-Resource-Agent modelingbasic template

Event

ResourceInternalagent

Location(if needed)

ExternalAgent

(if needed)

Event

ResourceInternalagent

Location(if needed)

ExternalAgent

(if needed)

These are allconsideredentities

Entity-Resource-Agent modeling Example

SellMerchandise

MerchandiseSalesperson

Customer

ReceiveCustomerpayment

Cash

CashRegister

decreases

increases

Takes place at

Takes place at

Collects payment

Sold by

Sold to

Received from

Resultsin

Now we add relations

Entity-Resource-Agent modelingwith diamonds

SellMerchandise

MerchandiseSalesperson

CashRegister

Customer

ReceiveCustomerpayment

Cash

decreases

increases

Takes place at

Collects payment

Sold by

Sold to

Received fromTakes place atR

esul

ts

in

Entity-Resource-Agent modeling

Entity

Relationship -Describes how two entities relate

Attribute -Specifies an entity (a record)

-Resource - such as merchandise or cash-Person (what we referred to as entities in DFDs)-Location (such as the cash register)

-Note that we never specified this before-Event

Entity-Relationship diagrams• There is a distinction between REA

modeling and E-R diagramming!– This distinction is not really important,

though.

• E-R diagrams can be used to graphically show the REA model

• This type of modeling is useful for designing databases

• Notice that the database/relationships design for the Ch03.mdb database looks very much like the ER diagram

Entity-Relationship modeling

Entity-Relationship modeling

tblCashDisbursementCheck No.

tblPurchaseOrderPO No.

tblCashDisbursementInventoryReceipt

Inv Rec No. + Chk No

tblInventoryReceiptInv Rec No

tblMaterialsInventoryInv. Stck No

tblVendorVendor No.

tblPOInventoryReceipt

PO No. + Inv Stck No.

CheckNo.

InvReceipt

No.

VendorNo.

PONo.

PONo.

InvStockNo.

Date

Inventory data

Vendor data

Entity-Relationship modeling

tblCashDisbursementCheck No.

tblPurchaseOrderPO No.

tblCashDisbursementInventoryReceipt

Inv Rec No. + Chk No

tblInventoryReceiptInv Rec No

tblMaterialsInventoryInv. Stck No

tblVendorVendor No.

tblPOInventoryReceipt

PO No. + Inv Stck No.

CheckNo.

InvReceipt

No.

VendorNo.

PONo.

PONo.

InvStockNo.

Date

Inventory data

Vendor data

Entity-Relationship modeling• Cardinality

– Within the context of ER modeling, we can characterize the cardinality of a relationship.

– Cardinality has to do with the number of possible outcomes that we are combining together

• Designations– 1-1 (one to one)

• This is when two tables are related and for every occurrence of the primary key in the first table, there is one and only one occurrence of the foreign key in the second table. Third normal form does not require any 1 - 1 relations

• Example:

Let’s rewrite this ONE table as two separate tables (like we did last class)

Entity-Relationship modeling

REGISTRATIONSSN LAST_NAME FIRST_NAME PHONE_NO LIC_PLATE_ST LIC_PLATE_NO123-34-5678 Curry Dorothy (916)358-4448 CA 123 MCD134-56-7783 Fong May (916)563-7865 CA 253 DAL

Example from last classNotice how each SSN is unique in the first AND the second table. If youknow any of the information in the table, you know it all. There are reasonsyou might want to design things this way though...

Person IDSSN LAST_NAME FIRST_NAME PHONE_NO123-34-5678 Curry Dorothy (916)358-4448134-56-7783 Fong May (916)563-7865

Plate IDLIC_PLATE_NO LIC_PLATE_ST SSN123 MCD CA 123-34-5678253 DAL CA 134-56-7783

Entity-Relationship modeling• Designations

– 1-1 (one to one)

Person ID Plate IDSSN

Entity-Relationship modeling• Designations

– 1-M (one to many)• This is the most common relationship• The primary key of the first table is unique in the

second table• Consider a customer table and an invoice table

– Each customer may have MANY invoices– Each invoice relates to ONLY ONE customer

tblCustomerCustNo.

tblInvoiceInvoiceNo.

CustNo.

Entity-Relationship modeling• Designations

– 1-M (one to many)• This is the most common relationship• The primary key of the first table is unique in the

second table• Consider a customer table and an invoice table

– Each customer may have MANY invoices– Each invoice relates to ONLY ONE customer

tblCustomerCustNo.

tblInvoiceInvoiceNo.

CustNo.

(1,M)

Entity-Relationship modeling• Designations

– M-M (many to many)• This is frequent in accounting contexts.• You have two tables

– In each table, there are multiple occurrences of the primary key of the other table

• Example is Invoices and Inventory Items– Each invoice may have several items in inventory– Each item in inventory may appear on several invoices

• The solution is to create a table that has a COMPOSITE PRIMARY KEY and build TWO relations

tblInventoryItemNo

tblInvoiceInvoiceNoItemNo. InvoiceNo.

tblInvoiceLineItemNo

InvoiceNo