REA analysis and E-R diagramming

REA analysisand E-R diagramming Part I April 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…

Location Event External Agent Resources Internal Agents Who What Where

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 Resource Internal agent Event Location (if needed) External Agent (if needed) Resource Internal agent These are all considered entities Event Location (if needed) External Agent (if needed)

Entity-Resource-Agent modelingExample Sold by decreases Sold to Takes place at Results in Received from Takes place at Collects payment increases Salesperson Merchandise Sell Merchandise Customer Cash Register Receive Customer payment Cash Now we add relations

Entity-Resource-Agent modelingwith diamonds Sold by decreases Sold to Takes place at Results in Received from Takes place at Collects payment increases Salesperson Merchandise Sell Merchandise Customer Cash Register Receive Customer payment Cash

Entity-Resource-Agent modeling • 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 • Describes how two entities relate Attribute • Specifies an entity (a record)

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. tblCashDisbursementInventoryReceipt Inv Rec No. + Chk No Check No. Date tblInventoryReceiptInv Rec No Inv Receipt No. PONo. tblPurchaseOrderPO No. tblPOInventoryReceiptPO No. + Inv Stck No. tblMaterialsInventoryInv. Stck No PONo. VendorNo. Inv Stock No. Inventory data tblVendorVendor No. 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:

Entity-Relationship modeling Example from last class Notice 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 reasons you might want to design things this way though... Let’s rewrite this ONE table as two separate tables (like we did last class)

Entity-Relationship modeling Person ID Plate ID SSN • Designations • 1-1 (one to one)

Entity-Relationship modeling tblCustomer CustNo. tblInvoiceInvoiceNo. CustNo. to designate many • 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

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 (1,M) tblCustomer CustNo. tblInvoiceInvoiceNo. CustNo. Alternate Approach

Entity-Relationship modeling tblInventory ItemNo tblInvoiceLine ItemNo InvoiceNo tblInvoiceInvoiceNo ItemNo. InvoiceNo. • 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

REA analysis and E-R diagramming